dt-bindings: ARM: Mediatek: Document bindings for MT2701

This patch adds the binding documentation for apmixedsys, bdpsys,
ethsys, hifsys, imgsys, infracfg, mmsys, pericfg, topckgen and
vdecsys for Mediatek MT2701.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Tested-by: John Crispin <blogic@openwrt.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>