dt-bindings: thermal: imx8mm-thermal: Add i.MX 8M Nano compatible

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/imx8mn-evk.dt.yaml: tmu@30260000:
    compatible:0: 'fsl,imx8mn-tmu' is not one of ['fsl,imx8mm-tmu', 'fsl,imx8mp-tmu']
    From schema: Documentation/devicetree/bindings/thermal/imx8mm-thermal.yaml

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: tmu@30260000:
    compatible: ['fsl,imx8mn-tmu', 'fsl,imx8mm-tmu'] is too long

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: tmu@30260000:
    compatible: Additional items are not allowed ('fsl,imx8mm-tmu' 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>