msm: iommu: Support mapping attributes

Add support for the IOMMU_READ and IOMMU_WRITE attributes
for mappings. Read-only and read-write mappings are fully
supported. Write-only mappings are not supported in
hardware and will fall back to read-write. Specifying
neither attribute in iommu_map will result in a warning on
the first call, and the mapping will be treated as
read-write.

Change-Id: Ibe9bd340d0743f37c91580c7db4f199fb4afba8b
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>