msm: adsprpc: Put upper limit on IOMMU mapping size

Place appropriate validation checks for IOMMU mapping size. Without
these checks, a mapping in an invalid kernel space could be created
and fail during access.

Change-Id: I995c8c2a9ef8d746b8fc2433d6510dfa92596adc
Signed-off-by: Jeya R <jeyr@codeaurora.org>