arch/powerpc/include/asm/io.h: fix warning: preprocessor token __iomem redefined

a fixup __iomem definition in arch code appears to be placed there as a cover
up from a code import from linux when u-boot didn't yet have a compiler.h,
introduced by commit 812711ce "Implement
__raw_{read,write}[bwl] on all architectures".

git show 812711ce:include/linux/compiler.h
fatal: Path 'include/linux/compiler.h' exists on disk, but not in '812711ce'.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>