Add driver for rpi-proto

Forward port of 3.10.x driver from https://github.com/koalo
We are using a custom board and would like to use rpi 3.18.x
kernel. Patch works fine for our embedded system.

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



Playback tested with devicetree enabled.

Signed-off-by: Waldemar Brodkorb <wbrodkorb@conet.de>