dt-bindings: gpio: fsl-imx-gpio: Add gpio-ranges property

The GPIO controller node can have gpio-ranges property.  This fixes
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: gpio@30200000: 'gpio-ranges' 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/20200825193536.7332-3-krzk@kernel.org