[media] videodev2.h: add SMPTE 2084 transfer function define

CEA-861.3 adds support for the SMPTE 2084 Electro-Optical Transfer Function
as can be used in HDR displays. Add a define for this in videodev2.h.

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