soc / drm: mediatek: Move mtk mutex driver to soc folder

mtk mutex is used by DRM and MDP driver, and its function is SoC-specific,
so move it to soc folder.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>