iommu: Introduce 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: I7ff6281b73a677c66051f63ff6f446c3b1659aa1
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>