dt-bindings: clock: mediatek: Add power-domains property

The mt8183-mfgcfg node uses a power domain in its device tree node.
To prevent schema validation warnings, add the optional `power-domains`
property to the binding schema for mediatek syscon clocks.

Fixes: 1781f2c4 ("arm64: dts: mediatek: mt8183: Add power-domains properity to mfgcfg")
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Julien Massot <julien.massot@collabora.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>