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.