media: rockchip/vpu: Add decoder boilerplate

This commit adds the needed boilerplate code to support the VPU
in decoding operation. Two v4l2 interfaces are exposed, one for
encoding and one for decoding, but a single m2m device is shared
by them, so jobs are properly serialized.

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>