iommu/arm-smmu: save the pgtbl_cfg in the domain

The pgtbl_cfg object has a few handy properties that we'd like to make
use of later (returning the pgd in a domain attribute, for example).
Keep track of the domain pgtbl_cfg in the domain structure.

Change-Id: Icce9638a270ca98d6ed6d61b3ef1c35d42a869dc
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>