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/src/main.c

@@ -23,9 +23,12 @@ int main(void)
 		return 0;
 	}
 	
-	// Test supply voltage reading
-	uint16_t supply_voltage = valve_get_supply_voltage();
-	LOG_INF("Supply voltage: %u mV", supply_voltage);
+	// Test supply voltage reading periodically
+	while (1) {
+		uint16_t supply_voltage = valve_get_supply_voltage();
+		LOG_INF("Supply voltage: %u mV", supply_voltage);
+		k_msleep(5000);  // Read every 5 seconds
+	}
 	
 	LOG_INF("Irrigation System Slave Node started successfully");
 	return 0;