mfd: rn5t618: Register power off callback optionally

Only register power off if the PMIC is defined as system power
controller (see Documentation/devicetree/bindings/power/
power-controller.txt).

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>