iio: ak8975: add ak09911 and ak09912 support

Add 2 new definition entries to support ak0991x compass.
Add a more advanced function to check we are dealing with the
expected device.
Remove standalone driver for ak09911.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>