ROCKCHIP ADC attached resistor ladder buttons
------------------------------------

Required properties:
 - compatible: "rockchip,key"
 - io-channels: Phandle to an ADC channel

Each button (key) is represented as a sub-node of "rockchip,key":

Required subnode-properties:
	- label: Descriptive name of the key.
	- linux,code: Keycode to emit.
	- rockchip,adc_value:: Voltage ADC input when this key is pressed.

Example:

#include <dt-bindings/input/input.h>

	rk_key: rockchip-key {
		compatible = "rockchip,key";
		status = "okay";

		io-channels = <&saradc 1>;

		vol-up-key {
			linux,code = <115>;
			label = "volume up";
			rockchip,adc_value = <1>;
		};

		vol-down-key {
			linux,code = <114>;
			label = "volume down";
			rockchip,adc_value = <170>;
		};

		power-key {
			gpios = <&gpio0 5 GPIO_ACTIVE_LOW>;
			linux,code = <116>;
			pinctrl-names = "default";
			pinctrl-0 = <&pwrbtn>;
			label = "power";
			gpio-key,wakeup;
		};

		menu-key {
			linux,code = <59>;
			label = "menu";
			rockchip,adc_value = <355>;
		};

		home-key {
			linux,code = <102>;
			label = "home";
			rockchip,adc_value = <746>;
		};

		back-key {
			linux,code = <158>;
			label = "back";
			rockchip,adc_value = <560>;
		};

		camera-key {
			linux,code = <212>;
			label = "camera";
			rockchip,adc_value = <450>;
		};
	};