V4L/DVB (11042): v4l2-api: Add definitions for V4L2_MPEG_STREAM_VBI_FMT_IVTV payloads

This addition to the v4l2-api add definitions for the constants and
data structures used for sliced VBI data insertion into MPEG streams triggered
by V4L2_MPEG_STREAM_VBI_FMT_IVTV.  This simply declares what the ivtv and
cx18 drivers and MythTV have already been doing and provides a proper data
structure definition to user space.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>