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

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

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

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,num_codes: Number of keycodes associated with the key
	- linux,press-times-secs: Times in seconds linked with different
				  keycodes
	- linux,key-codes: List of keycodes to be emitted with each detected
			   timed presses

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.
	- gpio-key,name: String, Input device name for this key.

Example nodes:

	gpio_timed_keys {
			compatible = "gpio-timed-keys";
			#address-cells = <1>;
			#size-cells = <0>;
			power {
				label = "Power";
				gpios = <&gpio1 0 1>;
				linux,num_codes = <2>;
				linux,press-time-secs = < 1 6>;
				linux,key-codes = <KEY_DOWN KEY_ENTER>;
				gpio-key,wakeup;
			};
			...