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/pgtable.c b/arch/ppc/mm/pgtable.c
index 6ea9185..a192487 100644
--- a/arch/ppc/mm/pgtable.c
+++ b/arch/ppc/mm/pgtable.c
@@ -39,7 +39,7 @@
 unsigned long ioremap_bot;
 int io_bat_index;
 
-#if defined(CONFIG_6xx) || defined(CONFIG_POWER3)
+#if defined(CONFIG_6xx)
 #define HAVE_BATS	1
 #endif