iommu/arm-smmu: add devices attached to the SMMU to an IOMMU group

IOMMU groups are expected by certain users of the IOMMU API,
e.g. VFIO. Add new devices found by the SMMU driver to an IOMMU
group to satisfy those users.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>