dt-bindings: gpio: fsl-imx-gpio: Add i.MX 8 compatibles

DTSes with new i.MX 8 SoCs introduce their own compatibles so add them
to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: gpio@30200000:
    compatible:0: 'fsl,imx8mm-gpio' is not one of ['fsl,imx1-gpio', 'fsl,imx21-gpio', 'fsl,imx31-gpio', 'fsl,imx35-gpio', 'fsl,imx7d-gpio']
    From schema: Documentation/devicetree/bindings/gpio/fsl-imx-gpio.yaml

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: gpio@30200000:
    compatible: ['fsl,imx8mm-gpio', 'fsl,imx35-gpio'] is too long

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: gpio@30200000:
    compatible: Additional items are not allowed ('fsl,imx35-gpio' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200825193536.7332-2-krzk@kernel.org