poweroff: Allow to register poweroff handlers ...

Allow to register handlers for poweroff. This allows to have multiple
poweroff implementations in a single binary. The implementation is close
to the restart handlers.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>