[media] v4l: vsp1: Move video device out of struct vsp1_rwpf

To make the video device nodes optional we need to decouple the [rw]pf
instances from the video devices. Move video devices out of struct
vsp1_rwpf and instantiate them dynamically in the core driver code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>