doc: reference: Add pages for audio

Add reference pages for the following APIs:

- Audio Codec
- DMIC

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

include/audio/codec.h

@@ -14,6 +14,13 @@
 #ifndef ZEPHYR_INCLUDE_AUDIO_CODEC_H_
 #define ZEPHYR_INCLUDE_AUDIO_CODEC_H_
 
+/**
+ * @brief Abstraction for audio codecs
+ *
+ * @defgroup audio_codec_interface Audio Codec Interface
+ * @{
+ */
+
 #include <drivers/i2s.h>
 
 #ifdef __cplusplus
@@ -217,4 +224,8 @@ static inline int audio_codec_apply_properties(struct device *dev)
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* ZEPHYR_INCLUDE_AUDIO_CODEC_H_ */

include/audio/dmic.h

@@ -7,9 +7,23 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/**
+ * @file
+ * @brief Public API header file for Digital Microphones
+ *
+ * This file contains the Digital Microphone APIs
+ */
+
 #ifndef ZEPHYR_INCLUDE_AUDIO_DMIC_H_
 #define ZEPHYR_INCLUDE_AUDIO_DMIC_H_
 
+/**
+ * @brief Abstraction for digital microphones
+ *
+ * @defgroup audio_dmic_interface Digital Microphone Interface
+ * @{
+ */
+
 #include <kernel.h>
 #include <device.h>
 
@@ -276,4 +290,8 @@ static inline int dmic_read(struct device *dev, u8_t stream, void **buffer,
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* ZEPHYR_INCLUDE_AUDIO_DMIC_H_ */