iio: hid-sensors: magnetometer : Add sensitivity

A number of Properties that can be applied to Data Fields are per data
field basis or for all data fields. Adding sensitivity field for all
magnetometer fields, which is most commonly used in currently available
sensor hubs.

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