ASoC: tegra-wm8903: Document new nvidia, headset property

Some devices have a 4-pin headset jack instead of 3-pin microphone jack.
The new boolean nvidia,headset property tells that the Mic Jack represents
the state of a headset microphone. This additional hardware description is
needed because microphone detection procedure differs in a case of a 4-pin
jack from a 3-pin jack.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200330204011.18465-2-digetx@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/nvidia,tegra-audio-wm8903.txt

@@ -29,6 +29,7 @@ Optional properties:
 - nvidia,hp-det-gpios : The GPIO that detect headphones are plugged in
 - nvidia,int-mic-en-gpios : The GPIO that enables the internal microphone
 - nvidia,ext-mic-en-gpios : The GPIO that enables the external microphone
+- nvidia,headset : The Mic Jack represents state of the headset microphone pin
 
 Example: