iommu: Add DOMAIN_ATTR_PT_BASE_ADDR domain attribute

This attribute can be used to get the base address of the page tables
for the given domain.  This can be useful for some drivers that switch
page tables around out-of-band (for example, per-process page tables
managed by the GPU).

Change-Id: I7c67c4c6435e5aadba129a53be532c2396dccf9c
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>