powerpc/kconfig: Kill PPC_MULTIPLATFORM

CONFIG_PPC_MULTIPLATFORM is a remain of the pre-powerpc days and isn't
really meaningful anymore. It was basically equivalent to PPC64 || 6xx.

This removes it along with the following changes:

 - 32-bit platforms that relied on PPC32 && PPC_MULTIPLATFORM now rely
   on 6xx which is what they want anyway.

 - A new symbol, PPC_BOOK3S, is defined that represent compliance with
   the "Server" variant of the architecture. This is set when either 6xx
   or PPC64 is set and open the door for future BOOK3E 64-bit.

 - 64-bit platforms that relied on PPC64 && PPC_MULTIPLATFORM now use
   PPC64 && PPC_BOOK3S

 - A separate and selectable CONFIG_PPC_OF_BOOT_TRAMPOLINE option is now
   used to control the use of prom_init.c

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/Makefile

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index dfec3d2..71901fb 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -75,7 +75,7 @@
 obj-$(CONFIG_PPC32)		+= entry_32.o setup_32.o
 obj-$(CONFIG_PPC64)		+= dma-iommu.o iommu.o
 obj-$(CONFIG_KGDB)		+= kgdb.o
-obj-$(CONFIG_PPC_MULTIPLATFORM)	+= prom_init.o
+obj-$(CONFIG_PPC_OF_BOOT_TRAMPOLINE)	+= prom_init.o
 obj-$(CONFIG_MODULES)		+= ppc_ksyms.o
 obj-$(CONFIG_BOOTX_TEXT)	+= btext.o
 obj-$(CONFIG_SMP)		+= smp.o