dt-bindings: leds: Add SC27xx breathing light controller documentation

This patch adds the binding documentation for Spreadtrum SC27xx series
breathing light controller, which supports 3 outputs: red LED, green
LED and blue LED.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>

Documentation/devicetree/bindings/leds/leds-sc27xx-bltc.txt

+LEDs connected to Spreadtrum SC27XX PMIC breathing light controller
+
+The SC27xx breathing light controller supports to 3 outputs:
+red LED, green LED and blue LED. Each LED can work at normal
+PWM mode or breath light mode.
+
+Required properties:
+- compatible: Should be "sprd,sc2731-bltc".
+- #address-cells: Must be 1.
+- #size-cells: Must be 0.
+- reg: Specify the controller address.
+
+Required child properties:
+- reg: Port this LED is connected to.
+
+Optional child properties:
+- label: See Documentation/devicetree/bindings/leds/common.txt.
+
+Examples:
+
+led-controller@200 {
+	compatible = "sprd,sc2731-bltc";
+	#address-cells = <1>;
+	#size-cells = <0>;
+	reg = <0x200>;
+
+	led@0 {
+		label = "red";
+		reg = <0x0>;
+	};
+
+	led@1 {
+		label = "green";
+		reg = <0x1>;
+	};
+
+	led@2 {
+		label = "blue";
+		reg = <0x2>;
+	};
+};