dt-bindings: iio: adc: ad7923: adjust documentation

- The ad7927 is fully compatible with ad7928 driver, add
documentation for device.
- ad7923 and ad7924 are treated the same in the driver, show
the relationship in the documentation.

Suggested-by: Lukasz Zemla <Lukasz.Zemla@woodward.com>
Signed-off-by: Edmund Berenson <edmund.berenson@emlix.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221122161437.18937-1-edmund.berenson@emlix.com


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