arm: mm: call __iommu_map_sg for unaligned buffers

For unaligned offset/size buffers given in the sg list from the client,
call the __iommu_map_sg which properly handles it.
Also support the per-buffer coherency in __iommu_map_sg.

Change-Id: I0c617465e005d4c25c1c54ba784e2082d9afcefb
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>