dt-bindings: panel: td028ttec1: add backlight property

This adds an additional backlight property as described
in panel-common.txt

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

Documentation/devicetree/bindings/display/panel/tpo,td028ttec1.txt

@@ -6,6 +6,7 @@ Required properties:
 
 Optional properties:
 - label: a symbolic name for the panel
+- backlight: phandle of the backlight device
 
 Required nodes:
 - Video port for DPI input
@@ -21,6 +22,7 @@ lcd-panel: td028ttec1@0 {
 	spi-cpha;
 
 	label = "lcd";
+	backlight = <&backlight>;
 	port {
 		lcd_in: endpoint {
 			remote-endpoint = <&dpi_out>;