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

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.com
Signed-off-by: Mark Brown <broonie@kernel.org>