iommu/arm-smmu: restrict virtual address range to 36 bits

There appear to be bugs in the 4-level support of io-pgtable-arm.  Work
around this by forcing all VAs to be 36 bits maximum so that we only
ever use 3 levels.

Change-Id: I5354afad05f74e12c51b86c97cdf1b2e86b68949
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>