msm: Add simple APIs to map contiguous buffer

Typicall, most iommu mapping happens through Ion or another
API. There may be other cases when clients need a simple
API to map a contiguous physical address into an iommu.
Add apis to support mapping and unmapping a physical range
into the iommu. The client is responsible for any reference
counting that needs to occur.

Change-Id: Ie5cb330417f7dd967748bb76cfdf9d123c579e04
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>