powerpc: Merge 32 and 64 bits asm-powerpc/io.h

The rework on io.h done for the new hookable accessors made it easier,
so I just finished the work and merged 32 and 64 bits io.h for arch/powerpc.

arch/ppc still uses the old version in asm-ppc, there is just too much gunk
in there that I really can't be bothered trying to cleanup.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>