arm64: dma-mapping: support per-buffer coherent mappings

For arm64 stage 1 mappings add support to either force a buffer to
be mapped as coherent through the DMA_ATTR_FORCE_COHERENT DMA
attribute, or to force a buffer to not be mapped as coherent by
using the DMA_ATTR_FORCE_NON_COHERENT DMA attribute.

Both the DMA_ATTR_FORCE_COHERENT and DMA_ATTR_FORCE_NON_COHERENT DMA
attributes override the buffer coherency configuration set by making
the device coherent.

Change-Id: I21be3a09874c9fcfc79c4dd408c827ef26f60f01
Signed-off-by: Liam Mark <lmark@codeaurora.org>