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>

arch/powerpc/include/asm/io.h

@@ -127,7 +127,6 @@ static inline void isync(void)
 /*
  * Non ordered and non-swapping "raw" accessors
  */
-#define __iomem
 #define PCI_FIX_ADDR(addr)	(addr)
 
 static inline unsigned char __raw_readb(const volatile void __iomem *addr)