docs: iio: ad7380: add adaq4370-4 and adaq4380-4

docs: iio: ad7380: add adaq4370-4 and adaq4380-4

Adding documentation for adaq4370-4 and adaq4380-4 supported devices. In
particular, document the reference voltage mechanism and the gain
parameter that are specific to adaq devices.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241030-ad7380-add-adaq4380-4-support-v4-5-864ff02babae@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>