nds32: implement the unmap_sg DMA operation

This matches the implementation of the more commonly used unmap_single
routines and the sync_sg_for_cpu method which should provide equivalent
cache maintainance.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Greentime Hu <greentime@andestech.com>
Tested-by: Greentime Hu <greentime@andestech.com>