iommu/arm-smmu: Mask TLBI address correctly

The less said about "~12UL" the better. Oh dear.

We get away with it due to calling constraints that mean IOVAs are
implicitly at least page-aligned to begin with, but still; oh dear.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>