Documentation: ABI: add specific icm42600 documentation

Hardware offset available as calibscale sysfs attributes are real
physical values exprimed in SI units.

calibscale_available sysfs attributes represents the range of
acceptable values.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>