arm64/io: Remove useless definition of mmiowb()

arm64 includes asm-generic/io.h, which provides a dummy definition of
mmiowb() if one isn't already provided by the architecture.

Remove the useless definition.

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