[media] videodev2.h: add V4L2_CTRL_FLAG_VOLATILE

Add a new VOLATILE control flag that is set for volatile controls.
That way applications know whether the value of the control is volatile
(i.e. can change continuously) or not.

Until now this was an internal property, but it is useful to know in
userspace as well.

A typical use case is the gain value when autogain is on. In that case the
hardware will continuously adjust the gain based various environmental
factors.

This patch just adds and documents the flag, it's not yet used.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>