nds32: use the generic remapping allocator for coherent DMA allocations

Replace the code that sets up uncached PTEs with the generic vmap based
remapping code.  It also provides an atomic pool for allocations from
non-blocking context, which we not properly supported by the existing
nds32 code.

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