dt-bindings: stm32: dfsdm: update audio properties

- Add missing compatible property in audio node.
- Remove obsolete "st,stm32-dfsdm-pdm" compatible.
- Remove useless comment in adc io-channels description.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201020155709.2621-2-olivier.moysan@st.com


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