docs: iio: add documentation for ad3552r driver

Add documentation for ad3552r driver, needed to describe the high-speed
driver debugfs attributes and shows how the user may use them.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250409-wip-bl-ad3552r-fixes-v5-2-fb429c3a6515@baylibre.com


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