ASoC: Provide ADC left/right channel source selection on WM8994

Allow the application to choose if the ADC data presented on the left
and right channels is sourced from the internal left or right channel.
This allows a mono recording to be presented as stereo on the external
bus.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>