Add dma_mmap_coherent() for goya and gaudi to match their use of
dma_alloc_coherent(), see the Link tag for why.

Link: https://lore.kernel.org/lkml/20200609091727.GA23814@lst.de/
Cc: Christoph Hellwig <hch@lst.de>
Cc: Zhang Li <li.zhang@bitmain.com>
Cc: Ding Z Nan <oshack@hotmail.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>