added softdevice audio sync timer

src/audio_sync_timer.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2023 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+#ifndef _AUDIO_SYNC_TIMER_H_
+#define _AUDIO_SYNC_TIMER_H_
+
+#include <zephyr/kernel.h>
+#include <stdint.h>
+
+/**
+ * @brief Capture a timestamp on the sync timer.
+ *
+ * @retval The current timestamp of the audio sync timer.
+ */
+uint32_t audio_sync_timer_capture(void);
+
+/**
+ * @brief Returns the last captured value of the sync timer.
+ *
+ * The captured time is corresponding to the I2S frame start.
+ *
+ * See @ref audio_sync_timer_capture().
+ *
+ * @retval The last captured timestamp of the audio sync timer.
+ */
+uint32_t audio_sync_timer_capture_get(void);
+
+#endif /* _AUDIO_SYNC_TIMER_H_ */