media: rockchip/vpu: Open-code media controller register

In preparation to support decoders, using a single memory-to-memory
device, we need to roll our own media controller entities registration.

To do that, we define a rockchip_vpu_func object that embeds the
video_device object plus all the elements that are needed to attach this
vdev to the media device.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>