iommu: msm: Add support for N-depth pre-fetch

N-depth prefetch is a feature of MMU500 allows
to set custom pre-fetch depth. This feature
brings run-time performance on SMMU translation.
Add support to enable that.

Change-Id: I82f7a1953764aba263ad6cdab2a6ff58bc4feb00
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>