arm64: dts: mediatek: mt8516: add auxadc node

Add node for the auxadc IP. The IP is compatible with the one found
in MT8173 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Link: https://lore.kernel.org/r/20201012205218.3010868-2-fparent@baylibre.com


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