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>

arch/nds32/kernel/dma.c

@@ -458,6 +458,12 @@ static void nds32_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 			       int nhwentries, enum dma_data_direction dir,
 			       unsigned long attrs)
 {
+	int i;
+
+	for (i = 0; i < nhwentries; i++, sg++) {
+		nds32_dma_sync_single_for_cpu(dev, sg_dma_address(sg),
+				sg->length, dir);
+	}
 }
 
 struct dma_map_ops nds32_dma_ops = {