MMUv3 initialization sequence.

The code in the initialize_mmu macro sets up MMUv3 memory mapping
identically to MMUv2 fixed memory mapping. Depending on
CONFIG_INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX symbol this code is
located in addresses it was linked for (symbol undefined), or not
(symbol defined), so it needs to be position-independent.

The code has the following assumptions:
  This code fragment is run only on an MMU v3.
  TLBs are in their reset state.
  ITLBCFG and DTLBCFG are zero (reset state).
  RASID is 0x04030201 (reset state).
  PS.RING is zero (reset state).
  LITBASE is zero (reset state, PC-relative literals); required to be PIC.

TLB setup proceeds along the following steps.

  Legend:
    VA = virtual address (two upper nibbles of it);
    PA = physical address (two upper nibbles of it);
    pc = physical range that contains this code;

After step 2, we jump to virtual address in the range 0x40000000..0x5fffffff
or 0x00000000..0x1fffffff, depending on whether the kernel was loaded below
0x40000000 or above. That address corresponds to next instruction to execute
in this code. After step 4, we jump to intended (linked) address of this code.
The scheme below assumes that the kernel is loaded below 0x40000000.

        Step0  Step1  Step2  Step3          Step4  Step5
        =====  =====  =====  =====          =====  =====
   VA      PA     PA     PA     PA     VA      PA     PA
 ------    --     --     --     --   ------    --     --
 E0..FF -> E0  -> E0  -> E0          F0..FF -> F0  -> F0
 C0..DF -> C0  -> C0  -> C0          E0..EF -> F0  -> F0
 A0..BF -> A0  -> A0  -> A0          D8..DF -> 00  -> 00
 80..9F -> 80  -> 80  -> 80          D0..D7 -> 00  -> 00
 60..7F -> 60  -> 60  -> 60
 40..5F -> 40         -> pc  -> pc   40..5F -> pc
 20..3F -> 20  -> 20  -> 20
 00..1F -> 00  -> 00  -> 00

The default location of IO peripherals is above 0xf0000000. This may be changed
using a "ranges" property in a device tree simple-bus node. See ePAPR 1.1, §6.5
for details on the syntax and semantic of simple-bus nodes. The following
limitations apply:

1. Only top level simple-bus nodes are considered

2. Only one (first) simple-bus node is considered

3. Empty "ranges" properties are not supported

4. Only the first triplet in the "ranges" property is considered

5. The parent-bus-address value is rounded down to the nearest 256MB boundary

6. The IO area covers the entire 256MB segment of parent-bus-address; the
   "ranges" triplet length field is ignored


MMUv3 address space layouts.
============================

Default MMUv2-compatible layout.

                      Symbol                   VADDR       Size
+------------------+
| Userspace        |                           0x00000000  TASK_SIZE
+------------------+                           0x40000000
+------------------+
| Page table       |                           0x80000000
+------------------+                           0x80400000
+------------------+
| KMAP area        |  PKMAP_BASE                           PTRS_PER_PTE *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
|                  |                                       (4MB * DCACHE_N_COLORS)
+------------------+
| Atomic KMAP area |  FIXADDR_START                        KM_TYPE_NR *
|                  |                                       NR_CPUS *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
+------------------+  FIXADDR_TOP              0xbffff000
+------------------+
| VMALLOC area     |  VMALLOC_START            0xc0000000  128MB - 64KB
+------------------+  VMALLOC_END
| Cache aliasing   |  TLBTEMP_BASE_1           0xc7ff0000  DCACHE_WAY_SIZE
| remap area 1     |
+------------------+
| Cache aliasing   |  TLBTEMP_BASE_2                       DCACHE_WAY_SIZE
| remap area 2     |
+------------------+
+------------------+
| Cached KSEG      |  XCHAL_KSEG_CACHED_VADDR  0xd0000000  128MB
+------------------+
| Uncached KSEG    |  XCHAL_KSEG_BYPASS_VADDR  0xd8000000  128MB
+------------------+
| Cached KIO       |  XCHAL_KIO_CACHED_VADDR   0xe0000000  256MB
+------------------+
| Uncached KIO     |  XCHAL_KIO_BYPASS_VADDR   0xf0000000  256MB
+------------------+


256MB cached + 256MB uncached layout.

                      Symbol                   VADDR       Size
+------------------+
| Userspace        |                           0x00000000  TASK_SIZE
+------------------+                           0x40000000
+------------------+
| Page table       |                           0x80000000
+------------------+                           0x80400000
+------------------+
| KMAP area        |  PKMAP_BASE                           PTRS_PER_PTE *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
|                  |                                       (4MB * DCACHE_N_COLORS)
+------------------+
| Atomic KMAP area |  FIXADDR_START                        KM_TYPE_NR *
|                  |                                       NR_CPUS *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
+------------------+  FIXADDR_TOP              0x9ffff000
+------------------+
| VMALLOC area     |  VMALLOC_START            0xa0000000  128MB - 64KB
+------------------+  VMALLOC_END
| Cache aliasing   |  TLBTEMP_BASE_1           0xa7ff0000  DCACHE_WAY_SIZE
| remap area 1     |
+------------------+
| Cache aliasing   |  TLBTEMP_BASE_2                       DCACHE_WAY_SIZE
| remap area 2     |
+------------------+
+------------------+
| Cached KSEG      |  XCHAL_KSEG_CACHED_VADDR  0xb0000000  256MB
+------------------+
| Uncached KSEG    |  XCHAL_KSEG_BYPASS_VADDR  0xc0000000  256MB
+------------------+
+------------------+
| Cached KIO       |  XCHAL_KIO_CACHED_VADDR   0xe0000000  256MB
+------------------+
| Uncached KIO     |  XCHAL_KIO_BYPASS_VADDR   0xf0000000  256MB
+------------------+


512MB cached + 512MB uncached layout.

                      Symbol                   VADDR       Size
+------------------+
| Userspace        |                           0x00000000  TASK_SIZE
+------------------+                           0x40000000
+------------------+
| Page table       |                           0x80000000
+------------------+                           0x80400000
+------------------+
| KMAP area        |  PKMAP_BASE                           PTRS_PER_PTE *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
|                  |                                       (4MB * DCACHE_N_COLORS)
+------------------+
| Atomic KMAP area |  FIXADDR_START                        KM_TYPE_NR *
|                  |                                       NR_CPUS *
|                  |                                       DCACHE_N_COLORS *
|                  |                                       PAGE_SIZE
+------------------+  FIXADDR_TOP              0x8ffff000
+------------------+
| VMALLOC area     |  VMALLOC_START            0x90000000  128MB - 64KB
+------------------+  VMALLOC_END
| Cache aliasing   |  TLBTEMP_BASE_1           0x97ff0000  DCACHE_WAY_SIZE
| remap area 1     |
+------------------+
| Cache aliasing   |  TLBTEMP_BASE_2                       DCACHE_WAY_SIZE
| remap area 2     |
+------------------+
+------------------+
| Cached KSEG      |  XCHAL_KSEG_CACHED_VADDR  0xa0000000  512MB
+------------------+
| Uncached KSEG    |  XCHAL_KSEG_BYPASS_VADDR  0xc0000000  512MB
+------------------+
| Cached KIO       |  XCHAL_KIO_CACHED_VADDR   0xe0000000  256MB
+------------------+
| Uncached KIO     |  XCHAL_KIO_BYPASS_VADDR   0xf0000000  256MB
+------------------+