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

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

Optional properties:
	- autorepeat: Boolean, Enable auto repeat feature of Linux input
	  subsystem.
	- label: String, name of the input device.

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

	- gpios: OF device-tree gpio specification.
	- interrupts: the interrupt line for that input.
	- label: Descriptive name of the key.
	- linux,code: Keycode to emit.

Note that either "interrupts" or "gpios" properties can be omitted, but not
both at the same time. Specifying both properties is allowed.

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.
	- wakeup-source: Boolean, button can wake-up the system.
			 (Legacy property supported: "gpio-key,wakeup")
	- wakeup-event-action: Specifies whether the key should wake the
	  system when asserted, when deasserted, or both. This property is
	  only valid for keys that wake up the system (e.g., when the
	  "wakeup-source" property is also provided).
	  Supported values are defined in linux-event-codes.h:
		EV_ACT_ASSERTED		- asserted
		EV_ACT_DEASSERTED	- deasserted
		EV_ACT_ANY		- both asserted and deasserted
	- linux,can-disable: Boolean, indicates that button is connected
	  to dedicated (not shared) interrupt which can be disabled to
	  suppress events from the button.

Example nodes:

	gpio-keys {
			compatible = "gpio-keys";
			autorepeat;

			up {
				label = "GPIO Key UP";
				linux,code = <103>;
				gpios = <&gpio1 0 1>;
			};

			down {
				label = "GPIO Key DOWN";
				linux,code = <108>;
				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;
			};
			...
