power: reset: gpio-poweroff: use sys-off handler API

Use the new sys-off handler API for gpio-poweroff. This allows us to
have more than one poweroff handler and prioritise them.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://lore.kernel.org/r/20231006130428.11259-3-francesco@dolcini.it


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