iommu/rockchip: Use OF_IOMMU to attach devices automatically

Converts the rockchip-iommu driver to use the OF_IOMMU infrastructure,
which allows attaching master devices to their IOMMUs automatically
according to DT properties.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>