led-backlight bindings

This binding is used to describe a basic backlight device made of LEDs.
It can also be used to describe a backlight device controlled by the output of
a LED driver.

Required properties:
  - compatible: "led-backlight"
  - leds: a list of LEDs

Optional properties:
  - brightness-levels: Array of distinct brightness levels. The levels must be
                       in the range accepted by the underlying LED devices.
                       This is used to translate a backlight brightness level
                       into a LED brightness level. If it is not provided, the
                       identity mapping is used.

  - default-brightness-level: The default brightness level.

Example:

	backlight {
		compatible = "led-backlight";

		leds = <&led1>, <&led2>;
		brightness-levels = <0 4 8 16 32 64 128 255>;
		default-brightness-level = <6>;
	};