tests: drivers: adc: adc_api: added nRF5340 Audio DK overlay

The adc_api test was missing an overlay file for the nRF5340
Audio Dk board.

Signed-off-by: Tim Sørensen <tims@demant.com>

tests/drivers/adc/adc_api/boards/nrf5340_audio_dk_nrf5340_cpuapp.overlay

@@ -0,0 +1,7 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2023 Benjamin Björnsson <benjamin.bjornsson@gmail.com>
+ */
+
+#include "nordic,nrf-saadc-common.dtsi"