include: media: Add vfl_devnode_type of VFL_TYPE_VIDEO

Upsstream are renaming VFL_TYPE_GRABBER to VFL_TYPE_VIDEO.
To make backporting the upstream Unicam driver easier, add an
extra enum entry (same as VFL_TYPE_GRABBER) to match that.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>