arch: Remove dummy mmiowb() definitions from arch code

Now that no driver code is using mmiowb() directly, remove the dummy
definitions remaining in architectures that don't make use of
asm-generic/io.h, as well as the definition in asm-generic/io.h itself.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>