ASoC: dapm: deprecate MICBIAS widget type

Commit 086d7f80 ("ASoC: Convert WM8962 MICBIAS to a supply widget",
2011-09-23) says:

	A supply widget is generally clearer than a MICBIAS widget and a
	mic bias is just a type of supply so use a supply widget for the
	MICBIAS. This also avoids confusion with the routing when
	connected to multiple inputs.

but this has never been documented as a policy.  Add some comments to
make it clear.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Mark Brown <broonie@kernel.org>