Device-Tree bindings for input/gpio_keys.c keyboard driver

Required properties:
	- compatible = "gpio-keys";

Optional properties:
	- input-name: input name of the device
	- autorepeat: Boolean, Enable auto repeat feature of Linux input
	  subsystem.
	- pinctrl-names : This should be defined if a target uses pinctrl framework.
			See "pinctrl" in Documentation/devicetree/bindings/pinctrl/msm-pinctrl.txt.
			It should specify the names of the configs that pinctrl can install in drive

			Following are the pinctrl configs that can be installed:
			"gpio_ts_active" : Active configuration of pins, this should specify active
			config defined in pin groups of interrupt and reset gpio.
			"gpio_ts_suspend" : Disabled configuration of pins, this should specify sleep
			config defined in pin groups of interrupt and reset gpio.
	- name		: input device name.
	- use-syscore	: use syscore functionality for driver.

Each button (key) is represented as a sub-node of "gpio-keys":
Subnode properties:

	- gpios: OF device-tree gpio specification.
	- label: Descriptive name of the key.
	- linux,code: Keycode to emit.

Optional subnode-properties:
	- linux,input-type: Specify event type this button/key generates.
	  If not specified defaults to <1> == EV_KEY.
	- debounce-interval: Debouncing interval time in milliseconds.
	  If not specified defaults to 5.
	- gpio-key,wakeup: Boolean, button can wake-up the system.

Example nodes:

	gpio_keys {
			compatible = "gpio-keys";
			#address-cells = <1>;
			#size-cells = <0>;
			autorepeat;
			input-name = "gpio-keys";
			pinctrl-names = "tlmm_gpio_key_active","tlmm_gpio_key_suspend";
			pinctrl-0 = <&gpio_key_active>;
			pinctrl-1 = <&gpio_key_suspend>;
			use-syscore;
			button@21 {
				label = "GPIO Key UP";
				linux,code = <103>;
				gpios = <&gpio1 0 1>;
			};