dt-bindings: Add a binding for Mediatek SCP

Add a DT binding documentation of SCP for the
MT8183 SoC from Mediatek.

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191112110330.179649-2-pihsun@chromium.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>