[media] videodev2.h: add defines for the VBI field start lines

While working with raw and sliced VBI support in several applications
I noticed that you really need to know the start linenumbers for
each video field in order to correctly convert the start line numbers
reported by v4l2_vbi_format to the line numbers used in v4l2_sliced_vbi_format.

This patch adds four defines that specify the start lines for each
field for both 525 and 625 line standards.

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