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>

drivers/iommu/rockchip-iommu.c

@@ -1407,9 +1407,4 @@ static struct platform_driver rk_iommu_driver = {
 		   .suppress_bind_attrs = true,
 	},
 };
-
-static int __init rk_iommu_init(void)
-{
-	return platform_driver_register(&rk_iommu_driver);
-}
-subsys_initcall(rk_iommu_init);
+builtin_platform_driver(rk_iommu_driver);