dt-bindings: gpu: mali-utgard: add optional power-domain reference

On some socs Mali Utgard gpus have both soc power-domains and external
supplying regulators, so add an optional power-domains property for
reference.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>