iommu/rockchip: : Use standard driver registration

It's been a long time since there was any reason to register IOMMU
drivers early. Convert to the standard platform driver helper.

CC: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/c08d58bff340da6a829e76d66d2fa090a9718384.1644005728.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>