ASoC: pcm3168a: Add support for multi DIN/DOUT with TDM slots parameter

The driver was wired to be only usable in DIN1/DOUT1 mode, switching
between TDM and non TDM modes based on the number of channels.

While keeping this functionality for compatibility add support for using
all DIN1/2/3/4 and DOUT1/2/3 if it is needed by setting the TDM slots to
2.

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