arm64: dts: imx8mq-librem5: add brightness levels to led-backlight

Add brightness-levels and default-brightness-level properties to
the librem5 board description that have been used for a long time.
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mq-librem5.dtsi

@@ -20,6 +20,8 @@ / {
 	backlight_dsi: backlight-dsi {
 		compatible = "led-backlight";
 		leds = <&led_backlight>;
+		brightness-levels = <255>;
+		default-brightness-level = <190>;
 	};
 
 	pmic_osc: clock-pmic {