mfd: stpmic1: Add PMIC poweroff via sys-off handler

Use devm_register_sys_off_handler() that allows to register multiple
power-off handlers.
This will allow boards using the stpmic1 to power-off.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230602062426.3947116-2-sean@geanix.com