staging:iio: Add common ADIS library

A lot of the devices from the ADIS family use the same methods for accessing
registers, sampling data and trigger handling. They also have similar register
layout for the control registers.

This patch adds a common library for these devices. The library implements
functions for reading and writing registers as buffer and trigger management. It
also provides a set functions for accessing the control registers and for
running the devices internal self-test. Having this common library code will
allow us to remove a lot of duplicated code.

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