dt-bindings: leds: Add binding for spi-byte LED.

This patch adds the binding documentation for a simple SPI based LED
controller which use only one byte for setting the brightness.

Signed-off-by: Christian Mauderer <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jacek Anaszewski <[e-mail]>

Documentation/devicetree/bindings/leds/leds-spi-byte.txt

···
+* Single Byte SPI LED Device Driver.
+
+The driver can be used for controllers with a very simple SPI protocol:
+- one LED is controlled by a single byte on MOSI
+- the value of the byte gives the brightness between two values (lowest to
+  highest)
+- no return value is necessary (no MISO signal)
+
+The value for lowest and highest brightness is dependent on the device and
+therefore on the compatible string.
+
+Depending on the compatible string some special functions (like hardware
+accelerated blinking) might can be supported too.
+
+The driver currently only supports one LED. The properties of the LED are
+configured in a sub-node in the device node.
+
+Required properties:
+- compatible: should be one of
+   * "ubnt,acb-spi-led"		microcontroller (SONiX 8F26E611LA) based device
+				used for example in Ubiquiti airCube ISP
+
+Property rules described in Documentation/devicetree/bindings/spi/spi-bus.txt
+apply.
+
+LED sub-node properties:
+- label:
+	see Documentation/devicetree/bindings/leds/common.txt
+- default-state:
+	see Documentation/devicetree/bindings/leds/common.txt
+	Only "on" and "off" are supported.
+
+Example:
+
+led-controller@0 {
+	compatible = "ubnt,acb-spi-led";
+	reg = <0>;
+	spi-max-frequency = <100000>;
+
+	led {
+		label = "white:status";
+		default-state = "on";
+	};
+};