tests: adc_api: add dts overlay for STM32F4-DISCO

Add a dts overlay file for the STM32F4-DISCO to enable ADC ch testing
with the adc_api test.

Signed-off-by: Ajith Anandhan <ajithanandhan0406@gmail.com>

Author: AjithAnandhan4

tests/drivers/adc/adc_api/boards/stm32f4_disco.overlay

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2025 Ajith Anandhan
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/{
+	zephyr,user {
+		io-channels = <&adc1 1>, <&adc1 6>;
+	};
+};
+
+&adc1 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	channel@1 {
+		reg = <1>;
+		zephyr,gain = "ADC_GAIN_1";
+		zephyr,reference = "ADC_REF_INTERNAL";
+		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
+		zephyr,resolution = <12>;
+	};
+
+	channel@6 {
+		reg = <6>;
+		zephyr,gain = "ADC_GAIN_1";
+		zephyr,reference = "ADC_REF_INTERNAL";
+		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
+		zephyr,resolution = <12>;
+	};
+};