arm64: dts: salvator-common: add 12V regulator to backlight

This fixes the warning "pwm-backlight backlight: backlight supply power
not found, using dummy regulator".

Fixes: b33be33 ("arm64: dts: salvator-x: Add panel backlight support")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/salvator-common.dtsi

@@ -62,6 +62,7 @@
 		brightness-levels = <256 128 64 16 8 4 0>;
 		default-brightness-level = <6>;
 
+		power-supply = <&reg_12v>;
 		enable-gpios = <&gpio6 7 GPIO_ACTIVE_HIGH>;
 	};
 
@@ -83,6 +84,15 @@
 		regulator-always-on;
 	};
 
+	reg_12v: regulator2 {
+		compatible = "regulator-fixed";
+		regulator-name = "fixed-12V";
+		regulator-min-microvolt = <12000000>;
+		regulator-max-microvolt = <12000000>;
+		regulator-boot-on;
+		regulator-always-on;
+	};
+
 	rsnd_ak4613: sound {
 		compatible = "simple-audio-card";