ASoC: dt-bindings: qcom: Document "aux-devs" property

In some cases we need to probe additional audio components that do
not appear as part of the DAI links specified in the device tree.
Examples for this are auxiliary devices such as analog amplifiers
or codecs.

To make them work they need to be added as part of "aux-devs"
and connected to some other audio component using the audio routes
configurable using "(qcom,)audio-routing".

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20200826095141.94017-2-stephan@gerhold.net
Signed-off-by: Mark Brown <broonie@kernel.org>