iommu: dma-mapping-fast: per-buffer coherent mappings

For "fast" 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: I465a88ee568fb6cd8206c26a62cbf02ac40328fa
Signed-off-by: Liam Mark <lmark@codeaurora.org>