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.com


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>