docs: iio: add documentation for adis16550 driver

Add documentation for adis16550 driver which describes the driver device
files and shows how the user may use the ABI for various scenarios
(configuration, measurement, etc.).

Co-developed-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Co-developed-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Signed-off-by: Robert Budai <robert.budai@analog.com>
Link: https://patch.msgid.link/20250217105753.605465-7-robert.budai@analog.com


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