dt-bindings: ARM: Mediatek: Document devicetree bindings for clock controllers

This adds the binding documentation for the mmsys, imgsys, vdecsys,
vencsys and vencltsys controllers found on Mediatek SoCs.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>