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/platforms/embedded6xx/Kconfig

@@ -1,6 +1,6 @@
 config EMBEDDED6xx
 	bool "Embedded 6xx/7xx/7xxx-based boards"
-	depends on PPC32 && BROKEN_ON_SMP && PPC_MULTIPLATFORM
+	depends on 6xx && BROKEN_ON_SMP
 
 config LINKSTATION
 	bool "Linkstation / Kurobox(HG) from Buffalo"