iio:imu: Add support for the ADIS16480 and similar IMUs

This patch adds support for the ADIS16375, ADIS16480, ADIS16485, ADIS16488 6
degree to 10 degree of freedom IMUs.

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