iio: imu: Add support for the ADIS16460 IMU

The ADIS16460 device is a complete inertial system that includes a triaxial
gyroscope and a triaxial accelerometer. It's more simplified design than
that of the ADIS16480, and does not offer the triaxial magnetometers &
pressure sensors. It does also have a temperature sensor (like the
ADIS16480).
Since it is part of the ADIS16XXX family, it re-uses parts of the ADIS
library.

Naturally, the register map is different and much more simplified than the
ADIS16480 subfamily, so it cannot be integrated into that driver. A major
difference is that the registers are not paged.

One thing that is particularly special about it, is that it requires a
higher delay between CS changes (i.e. when CS goes up, the spec recommends
that it be brought down after a minimum of 16 uS).
Other ADIS chips require (via spec) a minimum of 2 uS between CS changes.
The kernel's 10 uS default should be fine for those other chips; they
haven't been tested with lower CS change delays yet.

Datasheet:
  https://www.analog.com/media/en/technical-documentation/data-sheets/adis16460.pdf



Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>