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: f7356e47

 ("dt-bindings: iio: adc: ad7192: Add binding documentation for AD7192")
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>