nds32/io: Remove useless definition of mmiowb()

mmiowb() only makes sense for SMP platforms, so we can remove it
entirely for nds32.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>