iommu/tegra-smmu: Add iommu_group support

As the last step to making groups mandatory, clean up the remaining
drivers by adding basic support. Whilst it may not perfectly reflect
the isolation capabilities of the hardware (tegra_smmu_swgroup sounds
suspiciously like something that might warrant representing at the
iommu_group level), using generic_device_group() should at least
maintain existing behaviour with respect to the API.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>