arm64: dts: mediatek: Update mt81xx scpsys node to align with dt-bindings

Update scpsys nodes using simple-mfd in mt81xx SoC devicetree
to align with the bindings.

Add specific compatibles for syscon node, even it's a dummy compatible,
because syscon node must come with a specific compatible.

Remove the '#power-domain-cells" propertry since the simple-mfd node is
not the power domain provider; the provider is the child node.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Link: https://lore.kernel.org/r/20220811025813.21492-8-tinghan.shen@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>