staging: iio: ak8975: Add ak8975 magnetometer sensor

This is for the Asahi Kasei AK8975 3-axis magnetometer.  It resides within
the magnetometer section of the IIO subsystem, and implements the raw
magn_x,y,z_raw attributes, as well as magn_x,y,z_scale attributes for
converting to Gauss.

Changed to use the magn_x,y,z_scale attributes, and made those attributes
give the scale factor to conert to Gauss.  Also applied more fixes from
feedback.

Keeping mode as an unsigned long.  I'm going to add proper power management
at some point anyway, which would deprecate this attribute.

Signed-off-by: Andrew Chew <achew@nvidia.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>