dt-bindings: backlight: Add led-backlight binding

Add DT binding for led-backlight.
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/leds/backlight/led-backlight.txt

+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>;
+	};