iio: hid-sensor-gyro-3d: Add timestamp channel

Each sample has a timestamp field with this change. This timestamp may
be from the sensor hub when present or local kernel timestamp. And the
unit of timestamp is nanosecond.

Signed-off-by: Ye Xiang <xiang.ye@intel.com>
Link: https://lore.kernel.org/r/20210105093515.19135-3-xiang.ye@intel.com


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