LEDs connected to GPIO lines

Required properties:
- compatible : should be "gpio-leds".

Each LED is represented as a sub-node of the gpio-leds device.  Each
node's name represents the name of the corresponding LED.

LED sub-node properties:
- gpios :  Should specify the LED's GPIO, see "Specifying GPIO information
  for devices" in Documentation/powerpc/booting-without-of.txt.  Active
  low LEDs should be indicated using flags in the GPIO specifier.
- label :  (optional) The label for this LED.  If omitted, the label is
  taken from the node name (excluding the unit address).
- linux,default-trigger :  (optional) This parameter, if present, is a
  string defining the trigger assigned to the LED.  Current triggers are:
    "backlight" - LED will act as a back-light, controlled by the framebuffer
		  system
    "default-on" - LED will turn on
    "heartbeat" - LED "double" flashes at a load average based rate
    "ide-disk" - LED indicates disk activity
    "timer" - LED flashes at a fixed, configurable rate

Examples:

leds {
	compatible = "gpio-leds";
	hdd {
		label = "IDE Activity";
		gpios = <&mcu_pio 0 1>; /* Active low */
		linux,default-trigger = "ide-disk";
	};
};

run-control {
	compatible = "gpio-leds";
	red {
		gpios = <&mpc8572 6 0>;
	};
	green {
		gpios = <&mpc8572 7 0>;
	};
}
