dt-bindings: hwmon: ina2xx: Describe #io-channel-cells property

There are two drivers in the Linux kernel. One is hwmon based and second
IIO. IIO version requires to define #io-channel-cells to operate.
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/aa303b9fe3116e7f98d6b72822f7f57694366db3.1703077926.git.michal.simek@amd.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ti,ina2xx.yaml

@@ -32,6 +32,9 @@ properties:
   reg:
     maxItems: 1
 
+  "#io-channel-cells":
+    const: 1
+
   label:
     description: A descriptive name for this device.
 
@@ -80,6 +83,7 @@ examples:
         power-sensor@44 {
             compatible = "ti,ina220";
             reg = <0x44>;
+            #io-channel-cells = <1>;
             label = "vdd_3v0";
             shunt-resistor = <1000>;
             vs-supply = <&vdd_3v0>;