iommu: arm-smmu: Set context bank page table coherency

Currently the coherency of an iommu's page table is
dictated by the coherency of the iommu device.

Allow clients to force a context bank's page table as
coherent.

Change-Id: I89ad9b6033eb54be876f8c6bcddec35a148d840b
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>