ARM: dts: am437x-gp-evm: Add phandle for the backlight for the panel

With the backlight phandle the driver can manage the backlight on/off in
sync with the panel enable/disable.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/boot/dts/am437x-gp-evm.dts

@@ -55,7 +55,7 @@ vmmcwl_fixed: fixedregulator-mmcwl {
 		enable-active-high;
 	};
 
-	backlight {
+	lcd_bl: backlight {
 		compatible = "pwm-backlight";
 		pwms = <&ecap0 0 50000 PWM_POLARITY_INVERTED>;
 		brightness-levels = <0 51 53 56 62 75 101 152 255>;
@@ -86,6 +86,8 @@ lcd0: display {
 		compatible = "osddisplays,osd057T0559-34ts", "panel-dpi";
 		label = "lcd";
 
+		backlight = <&lcd_bl>;
+
 		panel-timing {
 			clock-frequency = <33000000>;
 			hactive = <800>;