iommu: Finish making iommu_group support mandatory

Now that all the drivers properly implementing the IOMMU API support
groups (I'm ignoring the etnaviv GPU MMUs which seemingly only do just
enough to convince the ARM DMA mapping ops), we can remove the FIXME
workarounds from the core code. In the process, it also seems logical to
make the .device_group callback non-optional for drivers calling
iommu_group_get_for_dev() - the current callers all implement it anyway,
and it doesn't make sense for any future callers not to either.

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