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: ddr-pmu@3d800000:
    compatible:0: 'fsl,imx8mm-ddr-pmu' is not one of ['fsl,imx8-ddr-pmu', 'fsl,imx8m-ddr-pmu', 'fsl,imx8mp-ddr-pmu']
    From schema: Documentation/devicetree/bindings/perf/fsl-imx-ddr.yaml

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: ddr-pmu@3d800000:
    compatible: ['fsl,imx8mm-ddr-pmu', 'fsl,imx8m-ddr-pmu'] is too long

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: ddr-pmu@3d800000:
    compatible: Additional items are not allowed ('fsl,imx8m-ddr-pmu' 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>