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/slave_node/boards/weact_stm32g431_core.overlay

@@ -25,10 +25,10 @@
 
 &adc1 {
     status = "okay";
-    pinctrl-0 = <&adc1_in1_pa0>;
+    pinctrl-0 = <&adc1_in1_pa0 &adc1_in15_pb0>;
     pinctrl-names = "default";
     st,adc-clock-source = "SYNC";
-    st,adc-prescaler = <4>;
+    st,adc-prescaler = <1>;
     #address-cells = <1>;
     #size-cells = <0>;
 
@@ -36,9 +36,18 @@
         reg = <1>;
         zephyr,gain = "ADC_GAIN_1";
         zephyr,reference = "ADC_REF_INTERNAL";
+        zephyr,acquisition-time = <ADC_ACQ_TIME_MAX>;  // Use maximum acquisition time for stability
+        zephyr,resolution = <12>;
+        zephyr,vref-mv = <2048>;  // STM32G431 VREFBUF at 2.048V
+    };
+
+    channel@15 {
+        reg = <15>;
+        zephyr,gain = "ADC_GAIN_1";
+        zephyr,reference = "ADC_REF_INTERNAL";
         zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
         zephyr,resolution = <12>;
-        zephyr,vref-mv = <3300>;
+        zephyr,vref-mv = <2048>;  // STM32G431 VREFBUF at 2.048V
     };
 };
 
@@ -47,4 +56,9 @@
     adc1_in1_pa0: adc1_in1_pa0 {
         pinmux = <STM32_PINMUX('A', 0, ANALOG)>; // PA0 in den Analogmodus setzen
     };
+
+    // Pinmux für PB0 als ADC1_IN15 (Analogmodus) - for lab supply testing
+    adc1_in15_pb0: adc1_in15_pb0 {
+        pinmux = <STM32_PINMUX('B', 0, ANALOG)>; // PB0 in den Analogmodus setzen
+    };
 };