dt-bindings: backlight: Add binding for RAVE SP backlight driver

Add Device Tree bindings for RAVE SP backlight driver - an MFD cell of
parent RAVE SP driver (documented in
Documentation/devicetree/bindings/mfd/zii,rave-sp.txt).
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/leds/backlight/zii,rave-sp-backlight.txt

+Zodiac Inflight Innovations RAVE Supervisory Processor Backlight Bindings
+
+RAVE SP backlight device is a "MFD cell" device corresponding to
+backlight functionality of RAVE Supervisory Processor. It is expected
+that its Device Tree node is specified as a child of the node
+corresponding to the parent RAVE SP device (as documented in
+Documentation/devicetree/bindings/mfd/zii,rave-sp.txt)
+
+Required properties:
+
+- compatible: Should be "zii,rave-sp-backlight"
+
+Example:
+
+	rave-sp {
+		compatible = "zii,rave-sp-rdu1";
+		current-speed = <38400>;
+
+		backlight {
+			compatible = "zii,rave-sp-backlight";
+		};
+	}
+