ASoC: Add driver for PROTO Audio CODEC (with a WM8731)

Add support for the MikroElektronika PROTO audio codec board.

URL to the audio chip:
http://www.mikroe.com/add-on-boards/audio-voice/audio-codec-proto/

Signed-off-by: Florian Meier <florian.meier@koalo.de>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>