iommu/rockchip: Don't feed NULL res pointers to devres

If we do, devres prints a "invalid resource" string in the error
loglevel.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>