iommu: msm: Make VFE SMMU conditionally secure

VFE SMMU needs to be secure for new secure camera use
cases. Change the VFE to be secure and designate the
last context bank (CB) as secure. Also ensure backwards
compatability so that when running with old secure environment
we fall back to VFE SMMU being non-secure.

Change-Id: I25f31b0350ef0c1b16ebb0db531cc0e6bc556fcf
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>