dt-bindings: nvmem: imx-ocotp: Update i.MX 8M compatibles

DTSes with new i.MX 8M SoCs use two compatibles so update the binding to
fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: efuse@30350000: compatible:1: 'syscon' was expected
    From schema: Documentation/devicetree/bindings/nvmem/imx-ocotp.yaml

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: efuse@30350000:
    compatible: ['fsl,imx8mn-ocotp', 'fsl,imx8mm-ocotp', 'syscon'] is too long

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: efuse@30350000:
    compatible: Additional items are not allowed ('syscon' 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>