iommu/omap: Convert to generic_single_device_group()

Use the new helper.

For some reason omap will probe its driver even if it doesn't load an
iommu driver. Keep this working by keeping a bool to track if the iommu
driver was started.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/7-v1-c869a95191f2+5e8-iommu_single_grp_jgg@nvidia.com


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