dt-bindings: media: mediatek,vcodec: Revise description

Revise the description of MediaTek video decoder to improve wording, fix
typos, simplify diagram, and extend the pipeline architecture used in
newer MediaTek SoCs (MT8186 and MT8188).

Signed-off-by: Fei Shao <fshao@chromium.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>