ASoC: SOF: sof-audio: Add support for loopback capture

ASoC: SOF: sof-audio: Add support for loopback capture

An example of a DAI-less loopback pipeline would be the echo
reference capture in the speaker playback path. This pipeline
is set up as follows:

Host(Playback) -> mixin -> mixout -> gain -> module-copier -> DAI
						|
						V
		Host(Capture) <-  Process module <- virtual DAI

In the above example, the virtual DAI exploits the concept of an
aggregated DAI (one with a non-zero DAI ID) in topology to enable this
pipeline to work with DPCM. A virtual DAI is a DAI widget with a
non-zero DAI ID and hence is skipped when traversing the list of DAPM
widgets during widget prepare/set/up/free/unprepare. The process module
in the above pipeline generates 0's that are captured by the echo
reference PCM.  When the playback path is active, the process module acts
as a passthrough module to allow the playback samples to be passthrough
to the capture host.

In order for these pipelines to work properly, the logic for
setting/preparing/freeing/unpreparing the widgets needs to be amended to
make sure that only the widgets that are in the pipeline in the same
direction as the PCM being started are set up. For example, when the
playback PCM is started, the capture pipeline widgets also show up in
the list of connected DAPM widgets but they shouldn't be set up yet
because the echo reference capture PCM hasn't been started yet.
Alternatively, when the echo reference capture PCM is started, the
playback pipeline widgets should not be setup.

Finally, the last step needed to put this all together is the set the
routes for widgets connecting the playback and the capture pipelines
when both are active.

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://patch.msgid.link/20260204081833.16630-9-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>