Add IQAudIO Digi WM8804 board support

Support IQAudIO Digi board with iqaudio_digi machine driver and
 iqaudio-digi-wm8804-audio overlay.

NB. Machine driver is a cut and paste of hifiberry_digi code, with format
    and general cleanup to comply with kernel coding standards.

Signed-off-by: DigitalDreamtime <clive.messer@digitaldreamtime.co.uk>