GPIO watchdog timer

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

Required properties:

- compatible: Must be "linux,wdt-gpio".
- gpios: From common gpio binding; gpio connection to WDT reset pin.
- hw_algo: The algorithm used by the driver. Should be one of the
  following values:
  - toggle: Toggle from high-to-low or low-to-high when resetting the watchdog.
  - level: Maintain a constant high/low level, and trigger a short pulse when
    resetting the watchdog. Active level is determined by the GPIO flags.
- 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";
		hw_algo = "toggle";
		always-running;
	};