dt-bindings: iio: adc: auxadc: add doc for MT8516 SoC

Add documentation for the auxadc binding for MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201012205218.3010868-1-fparent@baylibre.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>