GPIO watchdog timer

Describes a simple watchdog timer which is reset by toggling a gpio.

Required properties:

- compatible: Must be "linux,wdt-gpio".
- gpios: gpio to toggle when wdt driver reset method is called.
- always-running: Boolean property indicating that the watchdog cannot
  be disabled. At present, U-Boot only supports this kind of GPIO
  watchdog.

Example:

	gpio-wdt {
		gpios = <&gpio0 1 0>;
		compatible = "linux,wdt-gpio";
		always-running;
	};