ASoC: omap: n810: HS mic is not working, add a widget for it with comment

The bias for the analog HS microphone is coming from Retu/Vilma chip and
we do not have control over it, yet.

For clarity, add a new DAPM_MIC widget for the HS mic and document the
current state.

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