dt-bindings: reset: fsl,imx7-src: 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: reset-controller@30390000:
    compatible:0: 'fsl,imx8mm-src' is not one of ['fsl,imx7d-src', 'fsl,imx8mq-src', 'fsl,imx8mp-src']
    From schema: Documentation/devicetree/bindings/reset/fsl,imx7-src.yaml

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: reset-controller@30390000:
    compatible:1: 'syscon' was expected

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: reset-controller@30390000:
    compatible: ['fsl,imx8mm-src', 'fsl,imx8mq-src', 'syscon'] is too long

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