ppc: Remove CHRP, POWER3 and POWER4 support from arch/ppc

32-bit CHRP machines are now supported only in arch/powerpc, as are
all 64-bit PowerPC processors.  This means that we don't use
Open Firmware on any platform in arch/ppc any more.

This makes PReP support a single-platform option like every other
platform support option in arch/ppc now, thus CONFIG_PPC_MULTIPLATFORM
is gone from arch/ppc.  CONFIG_PPC_PREP is the option that selects
PReP support and is generally what has replaced
CONFIG_PPC_MULTIPLATFORM within arch/ppc.

_machine is all but dead now, being #defined to 0.

Updated Makefiles, comments and Kconfig options generally to reflect
these changes.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/ppc/mm/mmu_context.c b/arch/ppc/mm/mmu_context.c
index a8816e0..b4a4b3f 100644
--- a/arch/ppc/mm/mmu_context.c
+++ b/arch/ppc/mm/mmu_context.c
@@ -2,7 +2,7 @@
  * This file contains the routines for handling the MMU on those
  * PowerPC implementations where the MMU substantially follows the
  * architecture specification.  This includes the 6xx, 7xx, 7xxx,
- * 8260, and POWER3 implementations but excludes the 8xx and 4xx.
+ * 8260, and 83xx implementations but excludes the 8xx and 4xx.
  *  -- paulus
  *
  *  Derived from arch/ppc/mm/init.c: