iommu/rockchip: Use dev_iommu_priv_get/set()

Remove the use of dev->archdata.iommu and use the private per-device
pointer provided by IOMMU core code instead.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200625130836.1916-6-joro@8bytes.org