Add recording support to the Nuttx audio driver for Spresense.
- Supports 16 bit data with 48 kHz sample rate only for now.
- Supports 1 (dual mono) 2 or 4 channels.
- Only analog mics have been tested so digital is considered
unsupported.
Adds an initial Nuttx audio driver supporting the Spresense CXD56.
Being a work in progress the driver has a number of limitations:
- Audio playback only, no recording yet.
- Only 16 bit stereo playback is supported.
- In practice only 48kHz playback is supported due to missing SRC.
- Configure driver in "Device Drivers --> Audio Device Support".
