Device-Tree bindings for input/keyboard/imx_sc_pwrkey.c poweron/off driver
over SCU. On i.mx8QM/QXP poweron/off key is connected on SCU side, so need
to get key event by MU.

Required properties:
	- compatible = "fsl,imx8-pwrkey";

Each button/key looked as the sub node:
Required properties:
	- linux,code: the key value defined in
		include/dt-bindings/input/input.h
Optional property:
	- wakeup-source: wakeup feature, the keys can wakeup from
	suspend if the keys with this property pressed.

Example nodes:
	sc_pwrkey: sc-powerkey {
		compatible = "fsl,imx8-pwrkey";
		linux,keycode = <KEY_POWER>;
		wakeup-source;
	};