dt-bindings: backlight: Add led-backlight binding

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>;

	};