ARM: dts: s5pv210: add dummy 5V regulator for backlight on SMDKv210

Backlight is supplied by DC5V regulator.  The DTS has no PMIC node, so
just add a regulator-fixed to solve it and fix dtbs_check warning:

  s5pv210-smdkv210.dtb: backlight: 'power-supply' is a required property

Link: https://lore.kernel.org/r/20230421095721.31857-4-krzysztof.kozlowski@linaro.orgSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

arch/arm/boot/dts/s5pv210-smdkv210.dts

@@ -55,6 +55,14 @@ backlight {
 		default-brightness-level = <6>;
 		pinctrl-names = "default";
 		pinctrl-0 = <&pwm3_out>;
+		power-supply = <&dc5v_reg>;
+	};
+
+	dc5v_reg: regulator-0 {
+		compatible = "regulator-fixed";
+		regulator-name = "DC5V";
+		regulator-min-microvolt = <5000000>;
+		regulator-max-microvolt = <5000000>;
 	};
 };