Geomagnetic orientation(AM) sensor is one kind of orientation 6dof sensor.
It gives the device rotation in respect to the earth center and the
magnetic north. The sensor is implemented through use of an accelerometer
and magnetometer do not use gyroscope. It is a standard HID sensor.

More information can be found in:
http://www.usb.org/developers/hidpage/HUTRR59_-_Usages_for_Wearables.pdf

Geomagnetic orientation(AM) sensor and dev rotation sensor have same
channel and share channel usage id. So the most of the code for relative
orientation sensor can be reused.
Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>