iio: adis16260: Add ADIS16266 support

The ADIS16266 is mostly register compatible to the ADIS16260. The
difference is a different gyroscope scale factor as well not having the
relative angular displacement channel.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>