[media] videodev2.h: add COLORSPACE_DEFAULT

V4L2_COLORSPACE_DEFAULT is added so we have a specific define for
the default case where applications do not set it but leave it to 0.
In that case the driver will set the colorspace based on what it
captures.

This is already used, but we never had a define for the value 0.

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