ASoC: dapm - Add API call to query valid DAPM paths.

In preparation for ASoC DSP support.

Add a DAPM API call to determine whether a DAPM audio path is valid between
source and sink widgets. This also takes into account all kcontrol mux and mixer
settings in between the source and sink widgets to validate the audio path.

This will be used by the DSP core to determine the runtime DAI mappings
between FE and BE DAIs in order to run PCM operations.

Signed-off-by: Liam Girdwood <lrg@ti.com>