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

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

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

This also makes our data struct per-device and not global, which allows
for more than one instance of this device.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240212162831.67838-20-afd@ti.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>