dt-bindings: leds: Add bindings for lltc,lt3593

This patch adds the bindings document for LT3593 LED drivers.
The binding is kept consistent with other LED driver bindings in that it
stores all the LED-specific properties in its own subnode. As the hardware
only supports one channel, there can consequently only be one sub-node.
Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>

Documentation/devicetree/bindings/leds/leds-lt3593.txt

+Bindings for Linear Technologies LT3593 LED controller
+
+Required properties:
+- compatible:		Should be "lltc,lt3593".
+- lltc,ctrl-gpios:	A handle to the GPIO that is connected to the 'CTRL'
+			pin of the chip.
+
+The hardware supports only one LED. The properties of this LED are
+configured in a sub-node in the device node.
+
+Optional sub-node properties:
+- label:	A label for the LED. If none is given, the LED will be
+		named "lt3595::".
+- linux,default-trigger: The default trigger for the LED.
+			See Documentation/devicetree/bindings/leds/common.txt
+- default-state:	The initial state of the LED.
+			See Documentation/devicetree/bindings/leds/common.txt
+
+If multiple chips of this type are found in a design, each one needs to
+be handled by its own device node.
+
+Example:
+
+led-controller {
+	compatible = "lltc,lt3593";
+	lltc,ctrl-gpios = <&gpio 0 GPIO_ACTIVE_HIGH>;
+
+	led {
+		label = "white:backlight";
+		default-state = "on";
+	};
+};