[media] vidioc-enum-dv-timings.xml: explicitly state that pad and reserved should be zeroed

The ENUM_DV_TIMINGS documentation did not clearly state that the pad and reserved
fields should be zeroed (pad only when used with a video device node).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>