cleanup the dma_pgprot handling v2

From: Christoph Hellwig
Date: Mon Aug 26 2019 - 09:26:15 EST

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