iommu/arm-smmu: implement DOMAIN_ATTR_PT_BASE_ADDR using ttbr[0]

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 by returning ttbr[0] from the page table config.

Change-Id: Ie9b0241c7c7df18b25761bae79c1be1a283ff3a4
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>