dt-bindings: gpio: fsl-imx-gpio: add i.MX ARMv6 and ARMv7 compatibles

Several DTSes with ARMv6 and ARMv7 i.MX SoCs introduce their own
compatibles so add them to fix dtbs_check warnings like:

  arch/arm/boot/dts/imx35-pdk.dt.yaml: gpio@53fa4000:
    compatible: ['fsl,imx35-gpio', 'fsl,imx31-gpio'] is not valid under any of the given schemas

  arch/arm/boot/dts/imx51-babbage.dt.yaml: gpio@73f90000:
    compatible: ['fsl,imx51-gpio', 'fsl,imx35-gpio'] is not valid under any of the given schemas

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200920195848.27075-2-krzk@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>