dt-bindings: soc: add SCPSYS binding for MT7623 and MT7623A SoC

document the binding for enabling SCPSYS on MediaTek MT7623 and MT7623A
SoC. Where MT7623 SoC has the same definition about power domains with
MT2701, so it's fine to using MT2701 ones as MT7623's fallback.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>