[POWERPC] Create and use CONFIG_WORD_SIZE

Linus made this suggestion for the x86 merge and this starts the process
for powerpc.  We assume that CONFIG_PPC64 implies CONFIG_PPC_MERGE and
CONFIG_PPC_STD_MMU_32 implies CONFIG_PPC_STD_MMU.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>