Provide a default implementation of phys_to_dma and dma_to_phys in
mach-generic/dma_coherence.h.

If CONFIG_NEED_SG_DMA_LENGTH is defined, the dma_length field in
struct scatterlist is used. Set this up in mips_dma_map_sg so that
the default mips DMA ops can be used when SWIOTLB is enabled.
Signed-off-by: NJayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5409/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>