iommu/msm: Refactor device tree parsing

Query SMT and SID mapping information at probe-time instead
of attach-time to allow this information to be
error-checked at an earlier time.

Change-Id: Ib2bbdc8374f9c86c3e6013d298fe8b279b53d83b
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>