media: MAINTAINERS: add Yunfei Dong as mediatek vcodec driver maintainer

I have been working on mediatek driver development for a very long time,
and sent many patches to change the driver architecture. Add myself as
co-maintainer for mediatek vcodec driver.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>