dt-bindings: watchdog: fsl-imx-wdt: 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-var-som-symphony.dt.yaml: watchdog@30280000:
    compatible:0: 'fsl,imx8mm-wdt' is not one of ['fsl,imx21-wdt']
    From schema: Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml

  arch/arm64/boot/dts/freescale/imx8mm-var-som-symphony.dt.yaml: watchdog@30280000:
    compatible: ['fsl,imx8mm-wdt', 'fsl,imx21-wdt'] is too long

  arch/arm64/boot/dts/freescale/imx8mm-var-som-symphony.dt.yaml: watchdog@30280000:
    compatible: Additional items are not allowed ('fsl,imx21-wdt' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>