msm: iommu: Reorganize per-target device initialization

Organize the code to add IOMMU devices into per-target
blocks to make the code easier to maintain.

Change-Id: I8127b587d087ae2b05d3be5b207cd5e514e7247c
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>