ASoC: SOF: Support for echoref (virtual DAI)

Merge series from Peter Ujfalusi <peter.ujfalusi@linux.intel.com>:

The series adds support for echo reference functionality by allowing
the capturing of playback audio right before it leaves the DSP.
For this to work correctly we need a virtual DAI that is also connected
to the echo reference capture device and in absence of playback a
signal generator generates silence to allow the capture to run.
When the real playback starts, application will start to receive the
playback audio to be usable for echo reference.