dt-bindings: MediaTek: Add binding document for the AUXADC

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/soc/mediatek/auxadc.txt

+MediaTek AUXADC
+===============
+
+The Auxiliary Analog/Digital Converter (AUXADC) is an ADC found
+in some Mediatek SoCs which among other things measures the temperatures
+in the SoC. It can be used directly with register accesses, but it is also
+used by thermal controller which reads the temperatures from the AUXADC
+directly via its own bus interface. See
+Documentation/devicetree/bindings/thermal/mediatek-thermal.txt
+for the Thermal Controller which holds a phandle to the AUXADC.
+
+Required properties:
+- compatible: Must be "mediatek,mt8173-auxadc"
+- reg: Address range of the AUXADC unit
+
+Example:
+
+auxadc: auxadc@11001000 {
+	compatible = "mediatek,mt8173-auxadc";
+	reg = <0 0x11001000 0 0x1000>;
+};