msm: iommu: Increase maximum MID size to 5 bits

On msm8x60, the MID field on the AXI connection to the
IOMMU can be up to five bits wide. Thus, allow the IOMMU
context platform data to map up to 32 MIDs.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>