Define video buffer flags for different timestamp types. Everything up to
now have used either realtime clock or monotonic clock, without a way to
tell which clock the timestamp was taken from.
Also document that the clock source of the timestamp in the timestamp field
depends on buffer flags.

[mchehab@redhat.com: fix a few wrong references to Kernel 3.8 - as this patch
 is meant for 3.9]
Signed-off-by: NSakari Ailus <sakari.ailus@iki.fi>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>