dt: bindings: lp8860: Add trigger binding to the lp8860

Add a default trigger optional node to the child node.
This will allow the driver to set the trigger for a backlight.
Signed-off-by: NDan Murphy <dmurphy@ti.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJacek Anaszewski <jacek.anaszewski@gmail.com>

Documentation/devicetree/bindings/leds/leds-lp8860.txt

@@ -21,6 +21,8 @@ Required child properties:
 
 Optional child properties:
 	- label : see Documentation/devicetree/bindings/leds/common.txt
+	- linux,default-trigger :
+	   see Documentation/devicetree/bindings/leds/common.txt
 
 Example:
 
@@ -35,6 +37,7 @@ led-controller@2d {
 	led@0 {
 		reg = <0>;
 		label = "white:backlight";
+		linux,default-trigger = "backlight";
 	};
 }