power: reset: as3722-poweroff: Use devm_register_sys_off_handler(POWER_OFF)

Use device life-cycle managed register function to simplify probe and
exit paths.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20240212162831.67838-16-afd@ti.com


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