msm: iommu: Support the IOMMU_CACHE attribute

Instead of specifying a shareability attribute and a cache
policy, add support for the IOMMU_CACHE attribute to allow
cacheable mappings using the default memory cache policy.

Change-Id: I78442770e4e64fd72d9314d343223757593d3529
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>