iommu/fsl: Use iommu_group_get_for_dev() for IOMMU groups

Drop custom code and use IOMMU provided grouping support for PCI.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Varun Sethi <varun.sethi@freescale.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>