ARM: dts: meson: minix-neo-x8: define PMIC as power controller

The PMIC driver used to register itself as poweroff controller by
default, hence assuming that this device is using the PMIC as
system power controller.

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