iommu/io-pgtable-arm: Support SMMU coherent page tables

Some SMMUs can walk page tables in the CPU cache.  Enable this
behavior for SMMUs whose device tree node has the `coherent'
property set.

Change-Id: Icbb7a7eaf6c6e85c3ff4f981fe1dd1c3917228dc
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>