nds32: use generic ioremap

Use the generic ioremap_prot and iounmap helpers.

Note that the io.h include in pgtable.h had to be removed to not create
an include loop.  As far as I can tell there was no need for it to
start with.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Greentime Hu <green.hu@gmail.com>