iommu/rockchip: Make use of 'struct iommu_device'

Register hardware IOMMUs seperatly with the iommu-core code
and add a sysfs representation of the iommu topology.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>