cleanup the dma_pgprot handling v2

[Christoph Hellwig]

Hi all,

this series replaces the arch_dma_mmap_pgprot hooks with the
simpler pgprot_dmacoherent as used by the arm code already and
cleans up various bits around that area.

Changes since v1:
 - improve the new arm64 comment
 - keep the special DMA_ATTR_WRITE_COMBINE handling for mips and
   document it