Fix ADC devicetree compilation error for voltage divider

- Fix voltage divider devicetree configuration to reference ADC controller directly instead of channel node
- Switch from ADC API to sensor API for voltage divider usage
- Add required sensor and voltage divider configuration options
- Remove unnecessary zephyr,user node that was causing compilation issues
- The voltage divider now properly uses sensor framework and builds successfully

Hardware setup:
- Uses ADC1 channel 1 on pin PA0
- Voltage divider with 2.2kΩ output and 3.2kΩ total resistance
- Provides voltage readings through sensor API accounting for divider ratio

software/apps/adc_dt/boards/weact_stm32g431_core.overlay

@@ -1,28 +1,38 @@
 / {
-    zephyr,user {
-        io-channels = <&adc1 1>;
-        io-channel-names = "multi_sense";
-    };
+	vdd_sense: voltage-divider {
+		compatible = "voltage-divider";
+		/*
+		 * This reference must provide one argument (the channel number)
+		 * because of the "#io-channel-cells = <1>" in the &adc1 node.
+		 */
+		io-channels = <&adc1 1>;
+		output-ohms = <2200>;
+		full-ohms = <3200>;
+	};
 };
 
 &adc1 {
-    status = "okay";
+	status = "okay";
 
-    pinctrl-0 = <&adc1_in1_pa0>;
-    pinctrl-names = "default";
+	pinctrl-0 = <&adc1_in1_pa0>;
+	pinctrl-names = "default";
 
-    st,adc-clock-source = "SYNC";
-    st,adc-prescaler = <4>;
+	st,adc-clock-source = "SYNC";
+	st,adc-prescaler = <4>;
 
-    #address-cells = <1>;
-    #size-cells = <0>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+	/*
+	 * This line is required by the st,stm32-adc driver binding.
+	 * It declares that references to its channels need one extra argument.
+	 */
+	#io-channel-cells = <1>;
 
-    /* This defines channel 1 on adc1. The "1" in io-channels refers to this reg value. */
-    adc_channel_1: channel@1 {
-        reg = <1>;
-        zephyr,gain = "ADC_GAIN_1";
-        zephyr,reference = "ADC_REF_INTERNAL";
-        zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
-        zephyr,resolution = <12>;
-    };
+	adc_channel_1: channel@1 {
+		reg = <1>;
+		zephyr,gain = "ADC_GAIN_1";
+		zephyr,reference = "ADC_REF_INTERNAL";
+		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
+		zephyr,resolution = <12>;
+	};
 };