iio: imu: Add support for adis16475

Support ADIS16475 and similar IMU devices. These devices are
a precision, miniature MEMS inertial measurement unit (IMU) that
includes a triaxial gyroscope and a triaxial accelerometer. Each
inertial sensor combines with signal conditioning that optimizes
dynamic performance.

The driver adds support for the following devices:
* adis16470, adis16475, adis16477, adis16465, adis16467, adis16500,
  adis16505, adis16507.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>