dma-mapping: Introduce DMA_ATTR_IOMMU_USE_UPSTREAM_HINT

Add documentation for this new attribute. It will be used on the
skunk target for allowing smmu clients to set the bus attributes
required to use system cache.

Change-Id: Id0b0dc261e46082ac48accdeb949947dce375d28
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>