media: mediatek: vcodec: add description for vsi struct

The vsi (video shared information) struct needs to be synchronized
between firmware and host, as a change that is only done in the host
version of the struct but isn't synchronized to the firmware. This can
lead to decoding issues with H264 bitstreams. Highlight this requirement
within the struct descriptions.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>