media: rkvdec: Add the rkvdec driver

The rockchip vdec block is a stateless decoder that's able to decode
H264, HEVC and VP9 content. This commit adds the core infrastructure
and the H264 backend. Support for VP9 and HEVS will be added later on.

[mchehab+huawei@kernel.org: select MEDIA_CONTROLLER and REQUEST_API]
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>