dt-bindings: arm: fsl: Fix matching Purism Librem5 phones

All Purism Librem5 phones have three compatibles so they need their own
entry to fix dbts_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dt.yaml: /:
    compatible: ['purism,librem5r2', 'purism,librem5', 'fsl,imx8mq'] is not valid under any of the given schemas

  arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dt.yaml: /:
    compatible: ['purism,librem5r2', 'purism,librem5', 'fsl,imx8mq'] is too long

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