docs: iio: new docs for ad7380 driver

This adds a new page to document how to use the ad7380 ADC driver.

Credit: this docs is based on ad7944 docs.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20240709-ad7380-add-docs-v1-1-458ced3dfcc5@baylibre.com


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