ARM: dts: sun5i: Add backlight node to sun5i-q8-common.dtsi

All A13 based q8 formfactor tablets use the same backlight setup, add
a backlight devicetree node for controlling the backlight on these devices.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/boot/dts/sun5i-q8-common.dtsi

@@ -41,11 +41,21 @@
  */
 #include "sunxi-q8-common.dtsi"
 
+#include <dt-bindings/pwm/pwm.h>
+
 / {
 	aliases {
 		serial0 = &uart1;
 	};
 
+	backlight: backlight {
+		compatible = "pwm-backlight";
+		pwms = <&pwm 0 50000 PWM_POLARITY_INVERTED>;
+		brightness-levels = <0 10 20 30 40 50 60 70 80 90 100>;
+		default-brightness-level = <8>;
+		/* TODO: backlight uses axp gpio1 as enable pin */
+	};
+
 	chosen {
 		stdout-path = "serial0:115200n8";
 	};