dt-bindings: gpio: gpio-vf610: fix iMX 7ULP compatible matching

The i.MX 7ULP DTSes use two compatibles so update the binding to fix
dtbs_check warnings like:

  arch/arm/boot/dts/imx7ulp-com.dt.yaml: gpio@40ae0000:
    compatible: ['fsl,imx7ulp-gpio', 'fsl,vf610-gpio'] is too long

  arch/arm/boot/dts/imx7ulp-com.dt.yaml: gpio@40ae0000:
    compatible: Additional items are not allowed ('fsl,vf610-gpio' was unexpected)

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