powerpc: Don't compile in arch/ppc/kernel for 32-bit ARCH=powerpc

powerpc: Don't compile in arch/ppc/kernel for 32-bit ARCH=powerpc

All of the things needed for 32-bit ARCH=powerpc builds have now
moved to arch/powerpc/kernel, so we don't need to go down into
arch/ppc/kernel any more, and we can remove the CONFIG_PPC_MERGE
conditional from arch/ppc/kernel/Makefile.

There were two files still referenced in the merge section of
arch/ppc/kernel/Makefile: ppc-stub.o, depending on CONFIG_KGDB,
and dma-mapping.o, depending on CONFIG_NOT_COHERENT_CACHE.  None
of the platforms currently in ARCH=powerpc have caches that
aren't coherent with DMA, but when we do get one we'll move
dma-mapping.c over.  As for CONFIG_KGDB, none of the Kconfig
files in the tree define it, so I'll let it languish for now.

Signed-off-by: Paul Mackerras <paulus@samba.org>