nds32: Use asm-generic/io.h

Convert the nds32 architecture to make use of the new asm-generic/io.h
to provide address mapping functions. As the generic implementations are
suitable for nds32 this is primarily a matter of removing code.

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Macpaul Lin <macpaul@andestech.com>