iommu/arm-smmu: Change virtual address size limit to 39 bits

Restrict the virtual address size to 39-bits to allow
a maximum of 3 pagetable levels.

Change-Id: I264f23a2e16cc7599bdad8a161854bcf6e24dd4a
Signed-off-by: Sushmita Susheelendra <ssusheel@codeaurora.org>