dt-bindings: auxadc: Add binding document for Mediatek auxadc.

The commit adds the device tree binding documentation for the mediatek
auxadc found on Mediatek MT2701.
Thermal gets auxadc sample data by iio device.
So the commit changes auxadc device tree binding documentation from
/soc/mediatek/auxadc.txt to /iio/adc/mt6577_auxadc.txt.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NZhiyong Tao <zhiyong.tao@mediatek.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/soc/mediatek/auxadc.txt → Documentation/devicetree/bindings/iio/adc/mt6577_auxadc.txt

-MediaTek AUXADC
+* Mediatek AUXADC - Analog to Digital Converter on Mediatek mobile soc (mt65xx/mt81xx/mt27xx)
 ===============
 
 The Auxiliary Analog/Digital Converter (AUXADC) is an ADC found
@@ -10,12 +10,20 @@ 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
+  - compatible: Should be one of:
+    - "mediatek,mt2701-auxadc": For MT2701 family of SoCs
+    - "mediatek,mt8173-auxadc": For MT8173 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".
+  - #io-channel-cells: Should be 1, see ../iio-bindings.txt
 
 Example:
 
-auxadc: auxadc@11001000 {
-	compatible = "mediatek,mt8173-auxadc";
+auxadc: adc@11001000 {
+	compatible = "mediatek,mt2701-auxadc";
 	reg = <0 0x11001000 0 0x1000>;
+	clocks = <&pericfg CLK_PERI_AUXADC>;
+	clock-names = "main";
+	#io-channel-cells = <1>;
 };