[media] vidioc-g-dv-timings.rst: update v4l2_bt_timings struct

The new picture_aspect, cea861_vic and hdmi_vic fields were not documented,
even though the corresponding flags were.

Add documentation for these new fields.

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