Implement the ->device_group() and ->of_xlate() callbacks which are used
in order to group devices. Each group can then share a single domain.

This is implemented primarily in order to achieve the same semantics on
Tegra210 and earlier as on Tegra186 where the Tegra SMMU was replaced by
an ARM SMMU. Users of the IOMMU API can now use the same code to share
domains between devices, whereas previously they used to attach each
device individually.
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>