iommu: msm: Refactor code to separate compilation

Modify the code and update makefile and defconfig to allow us to
only compile one of the two IOMMU drivers that we have. Only one of
the drivers are needed at the same time. This will ensure we are not
compiling and linking code that is not needed.

Change-Id: I7db69f873245e57bddab6625e0b04e2ed48f44b6
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>