msm: kgsl: Cleanup iommu device tree probe

We only ever have two sub-devices for IOMMU, one device for the user
context and an optional entry for the secure context. Instead of
blindly querying for compatibles and try to match the string names,
explicitly search for the device names we need and set them up that
way.

This cleanup also lets us swap the behavior of the mmu.secured flag.
We assume it to be true and then clear it if the secure context is
not found rather than the other way around. This allows us to make
sure that the flag is set correctly during probe so we can skip
secure context setup.

Change-Id: Ic0dedbad540733a1eaf1729ddec0031f397d9e43
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>