ASoC: twl6040: Add control for HS and HF mono to stereo selection

The new controls will give user the ability to route the left PDM channel
data to the right headset/handsfree DAC.
HS mono to stereo switch: PDM channel 1 (or mono) data to both HS DAC.
HF mono to stereo switch: PDM channel 3 data to both HF DAC.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>