Add new channel types support for gravity sensor.
Gravity sensor provides an application-level or physical collection that
identifies a device that measures exclusively the force of Earth's
gravity along any number of axes.
More information can be found in:
http://www.usb.org/developers/hidpage/HUTRR59_-_Usages_for_Wearables.pdfSigned-off-by: Song Hongyan <email@example.com>
Signed-off-by: Jonathan Cameron <firstname.lastname@example.org>