[media] v4l: Helper function for obtaining timestamps

v4l2_get_timestamp() produces a monotonic timestamp but unlike
ktime_get_ts(), it uses struct timeval instead of struct timespec, saving
the drivers the conversion job when getting timestamps for v4l2_buffer's
timestamp field.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>