media: rockchip/vpu: Add infra to support MPEG-2 decoding

Only adds structs and helpers to allow supporting MPEG-2 decoding on
rockchip SoCs. Support for RK3399 and RK3288 will be added in separate
commits

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
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>