iommu/rockchip: Support sharing IOMMU between masters

There would be some masters sharing the same IOMMU device. Put them in
the same iommu group and share the same iommu domain.

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>