iio: hid-sensor-rotation: Add relative orientation sensor hid support

Relative orientation(AG) sensor is a 6dof orientation sensor,
it depends on acceleration and gyroscope sensor data. It gives
a quaternion describing the orientation of the device relative
to an initial orientation. It is a standard HID sensor.

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

Relative orientation(AG) sensor and dev rotation sensor have same
channels 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>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Xu Even <even.xu@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>