dt-bindings: iio: adc: add docs for AD7606C-{16,18} parts

dt-bindings: iio: adc: add docs for AD7606C-{16,18} parts

The driver will support the AD7606C-16 and AD7606C-18.
This change adds the compatible strings for these devices.

The AD7606C-16,18 channels also support these (individually configurable)
types of channels:
 - bipolar single-ended
 - unipolar single-ended
 - bipolar differential

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240919130444.2100447-8-aardelean@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>