fence: add missing descriptions for fence

The members child_list and active_list were added to the fence struct
without descriptions for the Documentation. Adding these.

Fixes: b55b54b5 ("staging/android: remove struct sync_pt")
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

include/linux/fence.h

@@ -49,6 +49,8 @@ struct fence_cb;
  * @timestamp: Timestamp when the fence was signaled.
  * @status: Optional, only valid if < 0, must be set before calling
  * fence_signal, indicates that the fence has completed with an error.
+ * @child_list: list of children fences
+ * @active_list: list of active fences
  *
  * the flags member must be manipulated and read using the appropriate
  * atomic ops (bit_*), so taking the spinlock will not be needed most