dt-bindings: serial: fsl-imx-uart: 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: pwm@30660000:
    compatible:0: 'fsl,imx8mm-pwm' is not one of ['fsl,imx1-pwm', 'fsl,imx27-pwm']
    From schema: Documentation/devicetree/bindings/pwm/imx-pwm.yaml

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: pwm@30660000:
    compatible: ['fsl,imx8mm-pwm', 'fsl,imx27-pwm'] is too long

  arch/arm64/boot/dts/freescale/imx8mm-evk.dt.yaml: pwm@30660000:
    compatible: Additional items are not allowed ('fsl,imx27-pwm' 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>