iommu/arm-smmu: implement DOMAIN_ATTR_PT_BASE_ADDR

Some clients need to get the base address of the page table for a given
IOMMU domain.  This functionality is provided by the
DOMAIN_ATTR_PT_BASE_ADDR domain attribute.  Implement it in the ARM SMMU
driver.

Change-Id: Ibe4b11a6eaf2ca42aec326c764777373cff769cc
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>