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

  arch/arm/boot/dts/imx7d-cl-som-imx7.dt.yaml: i2c@30a20000:
    compatible: ['fsl,imx7d-i2c', 'fsl,imx21-i2c'] is not valid under any of the given schemas (Possible causes of the failure):

  arch/arm/boot/dts/imx7d-cl-som-imx7.dt.yaml: i2c@30a20000:
    compatible: ['fsl,imx7d-i2c', 'fsl,imx21-i2c'] is too long
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>