dt-bindings: iio: chemical: sensirion,scd4x: Add yaml description

Add documentation for the SCD4x carbon dioxide sensor from Sensirion.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Roan van Dijk <roan@protonic.nl>
Link: https://lore.kernel.org/r/20211008101706.755942-2-roan@protonic.nl


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>