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_test/boards/weact_stm32g431_core.overlay

@@ -1,31 +1,8 @@
-/ {
-    zephyr,user {
-        io-channels = <&adc1 1>;
-        io-channel-names = "multisense";
-    };
-};
-
 &adc1 {
-    #address-cells = <1>;
-    #size-cells = <0>;
-    status = "okay";
-    st,adc-clock-source = "SYNC";
-    st,adc-prescaler = <4>;
     pinctrl-0 = <&adc1_in1_pa0>;
     pinctrl-names = "default";
+    status = "okay";
 
-    channel@1 {
-        reg = <1>;
-        zephyr,gain = "ADC_GAIN_1";
-        zephyr,reference = "ADC_REF_INTERNAL";
-        zephyr,acquisition-time = <ADC_ACQ_TIME_MAX>;
-        zephyr,resolution = <12>;
-        zephyr,vref-mv = <3300>;
-    };
+    st,adc-clock-source = "SYNC";
+    st,adc-prescaler = <4>;
 };
-
-&pinctrl {
-    adc1_in1_pa0: adc1_in1_pa0 {
-        pinmux = <STM32_PINMUX('A', 0, ANALOG)>;
-    };
-};