power: reset: syscon-poweroff: use builtin_platform_driver() to simplify code

The syscon_poweroff_register() doesn't do anything special, so it can use the
builtin_platform_driver() macro to eliminate boilerplate code.
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230807131951.3443880-6-yangyingliang@huawei.comSigned-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/reset/syscon-poweroff.c

@@ -93,9 +93,4 @@ static struct platform_driver syscon_poweroff_driver = {
 		.of_match_table = syscon_poweroff_of_match,
 	},
 };
-
-static int __init syscon_poweroff_register(void)
-{
-	return platform_driver_register(&syscon_poweroff_driver);
-}
-device_initcall(syscon_poweroff_register);
+builtin_platform_driver(syscon_poweroff_driver);