dt-bindings: gpio: fsl-imx-gpio: Add power-domains

Parse also optional power-domains property to fix dtbs_check warnings
like:

  arch/arm64/boot/dts/freescale/imx8qxp-ai_ml.dt.yaml: gpio@5d080000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
    From schema: Documentation/devicetree/bindings/gpio/fsl-imx-gpio.yaml

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200825193536.7332-5-krzk@kernel.org