xtensa: add HIGHMEM support

Introduce fixmap area just below the vmalloc region. Use it for atomic
mapping of high memory pages.
High memory on cores with cache aliasing is not supported and is still
to be implemented. Fail build for such configurations for now.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index df6f86c..17b3198 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -190,6 +190,24 @@
 
 	  If in doubt, say Y.
 
+config HIGHMEM
+	bool "High Memory Support"
+	help
+	  Linux can use the full amount of RAM in the system by
+	  default. However, the default MMUv2 setup only maps the
+	  lowermost 128 MB of memory linearly to the areas starting
+	  at 0xd0000000 (cached) and 0xd8000000 (uncached).
+	  When there are more than 128 MB memory in the system not
+	  all of it can be "permanently mapped" by the kernel.
+	  The physical memory that's not permanently mapped is called
+	  "high memory".
+
+	  If you are compiling a kernel which will never run on a
+	  machine with more than 128 MB total physical RAM, answer
+	  N here.
+
+	  If unsure, say Y.
+
 endmenu
 
 config XTENSA_CALIBRATE_CCOUNT