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

Required properties:
	- compatible = "gpio-keys-polled";
	- poll-interval: Poll interval time in milliseconds

Optional properties:
	- autorepeat: Boolean, Enable auto repeat feature of Linux input
	  subsystem.

Each button (key) is represented as a sub-node of "gpio-keys-polled":
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.
	- wakeup-source: Boolean, button can wake-up the system.

Example nodes:

	gpio_keys_polled {
			compatible = "gpio-keys-polled";
			#address-cells = <1>;
			#size-cells = <0>;
			poll-interval = <100>;
			autorepeat;
			button@21 {
				label = "GPIO Key UP";
				linux,code = <103>;
				gpios = <&gpio1 0 1>;
			};
			...