ASoC: wm8960: add DAC mono mix kcontrol

In normal operation, the left and right channel digital audio data is
converted to analogue in two separate DACs. There is a mono-mix mode
where the two audio channels are mixed together digitally and then
converted to analogue using only one DAC, while the other DAC is
switched off. The mono-mix signal can be selected to appear on both
analogue output channels. The mono mix is automatically attenuated by
6dB to prevent clipping.

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>