dt-bindings: watchdog: fsl-imx: document NXP compatibles

Document all ARMv5, ARMv6, ARMv7 and ARMv8 NXP (i.MX, Layerscape)
compatibles used in DTSes (even though driver binds only to
fsl,imx21-wdt) to fix dtbs_check warnings like:

  arch/arm/boot/dts/imx53-qsb.dt.yaml: gpio@53fe0000: compatible:
    ['fsl,imx53-gpio', 'fsl,imx35-gpio'] is not valid under any of the given schemas

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