ASoC: tegra-wm8903: Document built-in microphone audio source

The internal microphone source is needed in order to be able to describe
the hardware audio routing for devices that have the built-in microphone
in addition to the external Mic Jack.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://lore.kernel.org/r/20200320205504.30466-2-digetx@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -18,6 +18,7 @@ Required properties:
   * Headphone Jack
   * Int Spk
   * Mic Jack
+  * Int Mic
 
 - nvidia,i2s-controller : The phandle of the Tegra I2S1 controller
 - nvidia,audio-codec : The phandle of the WM8903 audio codec