ARM: asm/io.h: kill off confusing #ifdef __mem_pci block

No ARM board seems to define __mem_pci - and if it did, one would get tons of

  ./arch/arm/include/asm/io.h:307:0: warning: "readl" redefined

warnings, because readl and friends are unconditionally defined
earlier in io.h. Moreover, the redefinitions lack the memory barriers
that the first definitions have. So I'm guessing this is practically
dead code.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>