sound: ASoC: DAPM support for ADC on WM9713 PCM interface

The stereo ADC in the WM9713 can be used to produce data for both the
standard AC97 interface and the additional voice PCM interface. Support
use on both by defining virtual ADCs tied to each accepting the output
from the actual ADCs.

Reported-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>