dt-bindings: ARM: Mediatek: Document bindings for MT2712

This patch adds the binding documentation for apmixedsys, bdpsys,
imgsys, imgsys, infracfg, mcucfg, mfgcfg, mmsys, pericfg, topckgen,
vdecsys and vencsys for Mediatek MT2712.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>