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

Documentation/devicetree/bindings/iio/adc/mt6577_auxadc.txt

@@ -17,6 +17,7 @@ Required properties:
     - "mediatek,mt7622-auxadc": For MT7622 family of SoCs
     - "mediatek,mt8173-auxadc": For MT8173 family of SoCs
     - "mediatek,mt8183-auxadc", "mediatek,mt8173-auxadc": For MT8183 family of SoCs
+    - "mediatek,mt8516-auxadc", "mediatek,mt8173-auxadc": For MT8516 family of SoCs
   - reg: Address range of the AUXADC unit.
   - clocks: Should contain a clock specifier for each entry in clock-names
   - clock-names: Should contain "main".