dt-bindings: pwm: imx: document i.MX compatibles

Document all ARMv5, ARMv6 and ARMv7 i.MX compatibles to fix dtbs_check
warnings like:

  arch/arm/boot/dts/imx6dl-colibri-eval-v3.dt.yaml: pwm@2080000: compatible:0:
    'fsl,imx6q-pwm' is not one of ['fsl,imx8mm-pwm', 'fsl,imx8mn-pwm', 'fsl,imx8mp-pwm', 'fsl,imx8mq-pwm']

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200925212609.23093-1-krzk@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>