dt-bindings: iio: afe: current-sense-shunt: add io-channel-cells

The current-sense-shunt is an IIO provider thus can be referenced by IIO
consumers (via "io-channels" property in consumer device node).
Such provider is required to describe number of cells used in phandle
lookup with "io-channel-cells" property.  This also fixes dtbs_check
warnings like:

  arch/arm/boot/dts/s5pv210-fascinate4g.dt.yaml: current-sense-shunt:
    '#io-channel-cells' does not match any of the regexes: 'pinctrl-[0-9]+'

Fixes: ce66e52b ("dt-bindings:iio:afe:current-sense-shunt: txt to yaml conversion.")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/afe/current-sense-shunt.yaml

@@ -24,6 +24,9 @@ properties:
     description: |
       Channel node of a voltage io-channel.
 
+  "#io-channel-cells":
+    const: 0
+
   shunt-resistor-micro-ohms:
     description: The shunt resistance.
 
@@ -57,6 +60,7 @@ examples:
     sysi {
         compatible = "current-sense-shunt";
         io-channels = <&tiadc 0>;
+        #io-channel-cells = <0>;
 
         /* Divide the voltage by 3300000/1000000 (or 3.3) for the current. */
         shunt-resistor-micro-ohms = <3300000>;