Documentation: iio: ad7380: add AD7389-4

Document support for AD7389-4 and it's quirks.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250401-iio-ad7380-add-ad7389-4-v1-5-23d2568aa24f@baylibre.com


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