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>