[media] v4l2-dv-timings: log if the timing is reduced blanking V2

The last CVT standard introduced reduced blanking version 2 which is signaled by
a vsync of 8. Log this.

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