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
Signed-off-by: Paul Mackerras <firstname.lastname@example.org>