dt-bindings: iio: ad7192: Fix DTC warning in the example

The example contains an SPI bus and device, but doesn't have the
appropriate size and address cells size.

This creates a DTC warning when the example is compiled since the default
ones will not match what the device uses. Let's add them to remove that
warning.

Fixes: f7356e47032c ("dt-bindings: iio: adc: ad7192: Add binding documentation for AD7192")
Signed-off-by: Maxime Ripard <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/iio/adc/adi,ad7192.yaml

···
 examples:
   - |
     spi0 {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
       adc@0 {
         compatible = "adi,ad7192";
         reg = <0>;