dt-bindings: arm: fsl: document i.MX25 and i.MX27 boards

Document and adjust the compatibles for i.MX25 and i.MX27 based boards
to fix dtbs_check warnings like:

  arch/arm/boot/dts/imx27-apf27dev.dt.yaml: /: compatible:
    ['armadeus,imx27-apf27dev', 'armadeus,imx27-apf27', 'fsl,imx27'] 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>