iommu/arm-smmu: Remove "leaf" indirection

Now that the "leaf" flag is no longer part of an external interface,
there's no need to use it to infer a register offset at runtime when
we can just as easily encode the offset directly in its place.

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