iio: imu: adis16480: Add docs for ADIS16480 IMU

Document support for ADIS16480 Inertial Measurement Unit.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>