iommu: Remove domain_init and domain_free iommu_ops

All drivers have been converted to the new domain_alloc and
domain_free iommu-ops. So remove the old ones and get rid of
iommu_domain->priv too, as this is no longer needed when the
struct iommu_domain is embedded in the private structures of
the iommu drivers.

Tested-by: Thierry Reding <treding@nvidia.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>