iommu: Revive device_group iommu-ops call-back

That call-back is currently unused, change it into a
call-back function for finding the right IOMMU group for a
device.
This is a first step to remove the hard-coded PCI dependency
in the iommu-group code.

Signed-off-by: Joerg Roedel <jroedel@suse.de>