iio:Documentation: Add documentation for hinge sensor channels

Add channel description for hinge sensor, including channel label
attribute and raw data description.

Signed-off-by: Ye Xiang <xiang.ye@intel.com>
Link: https://lore.kernel.org/r/20201215054444.9324-4-xiang.ye@intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>