dt-bindings: soc: update the binding document for SCPSYS on MediaTek MT7622 SoC

Update the binding document for enabling SCPSYS on MediaTek MT7622 SoC.

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