iommu: msm: Refactor page table code

Move code that is specific to short descriptor page table
format to its own file to allow sharing of header file
between long descriptor format page table and short descriptor
page table format.

Change-Id: Ia072a77a23d21207edb983de47b6fe69029ffdd7
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>