iommu/tegra-smmu: Add debugfs support

Provide clients and swgroups files in debugfs. These files show for
which clients IOMMU translation is enabled and which ASID is associated
with each SWGROUP.

Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>