dt-bindings: mtd: gpmi-nand: Add i.MX 8M compatibles

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

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: nand-controller@33002000:
    compatible:0: 'fsl,imx8mm-gpmi-nand' is not one of ['fsl,imx23-gpmi-nand', 'fsl,imx28-gpmi-nand', 'fsl,imx6q-gpmi-nand', 'fsl,imx6sx-gpmi-nand', 'fsl,imx7d-gpmi-nand']
    From schema: Documentation/devicetree/bindings/mtd/gpmi-nand.yaml

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: nand-controller@33002000:
    compatible: ['fsl,imx8mm-gpmi-nand', 'fsl,imx7d-gpmi-nand'] is too long

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: nand-controller@33002000:
    compatible: Additional items are not allowed ('fsl,imx7d-gpmi-nand' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>