dt-bindings: interrupt-controller: fsl,irqsteer: Fix compatible matching

The i.MX 8M DTSes use two compatibles so update the binding to fix
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mq-thor96.dt.yaml: interrupt-controller@32e2d000:
    compatible: ['fsl,imx8m-irqsteer', 'fsl,imx-irqsteer'] is too long
    From schema: Domentation/devicetree/bindings/interrupt-controller/fsl,irqsteer.yaml

  arch/arm64/boot/dts/freescale/imx8mq-thor96.dt.yaml: interrupt-controller@32e2d000:
    compatible: Additional items are not allowed ('fsl,imx-irqsteer' 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>