Device-Tree bindings for input/keyboard/rpmsg-keys.c keys driver over
rpmsg. On i.mx7ULP keys are connected on M4 side, so rpmsg-keys driver
needed to get the key status from M4 side by rpmsg.

Required properties:
	- compatible = "fsl,rpmsg-keys";

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

Example nodes:
	rpmsg_keys: rpmsg-keys {
		compatible = "fsl,rpmsg-keys";

		volume-up {
			label = "Volume Up";
			rpmsg-key,wakeup;
			linux,code = <KEY_VOLUMEUP>;
		};

		volume-down {
			label = "Volume Down";
			rpmsg-key,wakeup;
			linux,code = <KEY_VOLUMEDOWN>;
		};
	};