[media] videodev2.h: add enum/query/cap dv_timings ioctls

These new ioctls make it possible for the dv_timings API to replace
the dv_preset API eventually.

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