Document: DT: Add bindings for mediatek MT2701 SoC Platform

This adds a DT binding documentation for the MT2701 SoC from Mediatek.

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>