dt-bindings: arm: Add bindings for Mediatek MT2712 SoC Platform

This adds dt-binding documentation for Mediatek MT2712.
Only include very basic items: cpu, gic and uart.

Signed-off-by: YT Shen <yt.shen@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>