iommu: msm: Calculate context bank base address

Typically Context bank base address are 0x8000 apart from
global register address space. In few cases, they are not.
Consider those few cases and calculate the context bank
register offset and number accordingly.

Change-Id: Ibe49fc604af981543575e61176f676a60abd16dd
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>