iommu/io-pgtable-arm: Support non-coherent stage-2 page tables

Commit 9e6ea59f ("iommu/io-pgtable: Support non-coherent page tables")
added support for non-coherent page-table walks to the Arm IOMMU page-table
backends. Unfortunately, it left the stage-2 allocator unchanged, so let's
hook that up in the same way.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>