nds32: consolidate DMA cache maintainance routines

Make sure all other DMA methods call nds32_dma_sync_single_for_{device,cpu}
to perform cache maintaince, and remove the consisteny_sync helper that
implemented both with entirely separate code based off an argument.

Also make sure these helpers handled highmem properly, for which code
is copy and pasted from mips.

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