iommu/tegra-smmu: Allow to group clients in same swgroup

There can be clients using the same swgroup in DT, for example i2c0
and i2c1. The current driver will add them to separate IOMMU groups,
though it has implemented device_group() callback which is to group
devices using different swgroups like DC and DCB.

All clients having the same swgroup should be also added to the same
IOMMU group so as to share an asid. Otherwise, the asid register may
get overwritten every time a new device is attached.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200911071643.17212-4-nicoleotsuka@gmail.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>