ASoC: sgtl5000: add 'Audio Switch' block implementation

Add mux for choosing DAC source: ADC, I2S or DAP. DAP - digital
audio processing block. Moreover create choosing mux for DAP main
source (ADC or I2S) and DAP mixer source (ADC or I2S). Mixer block
allows summing DAP main and mixer sources togheter with defined
volumes.

Signed-off-by: Michal Oleszczyk <oleszczyk.m@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>