media: mtk-vcodec: Separating mtk encoder driver

MTK H264 Encoder(VENC_SYS) and VP8 Encoder(VENC_LT_SYS) are two
independent hardware instance. They have their owner interrupt,
register mapping, and special clocks.

This patch separates them into two devices. This is a preparing
patch for adding device_link between the larbs and venc-device.
It's mainly for fixing the problem:
https://lkml.org/lkml/2019/9/3/316



Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Maoguang Meng <maoguang.meng@mediatek.com>
Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>