dt-bindings: mfd: aspeed,ast2x00-scu: Convert to DT schema format

Convert the aspeed,ast2[456]00-scu binding to DT schema format.

The original binding was missing '#address-cells', '#size-cells',
'ranges', and child nodes, so add them.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220810161635.73936-3-robh@kernel.org