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.).

Signed-off-by: Ramona Gradinariu <[email protected]>
Signed-off-by: Antoniu Miclaus <[email protected]>

Author: rbolboac