[media] Rename V4L2_(IN|OUT)_CAP_CUSTOM_TIMINGS

The 'custom' timings are no longer just for custom timings, but also for standard
CEA/VESA timings. So rename to V4L2_IN/OUT_CAP_DV_TIMINGS.
The old define is still kept for backwards compatibility.
This decision was taken during the 2012 Media Workshop.

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