Currently the dtbs_check for imx6 generates warnings like this:

'fsl,imx6sl-dcp' is not one of ['fsl,imx23-dcp', 'fsl,imx28-dcp']
['fsl,imx6sl-dcp', 'fsl,imx28-dcp'] is too long

or

'fsl,imx6ull-dcp' is not one of ['fsl,imx23-dcp', 'fsl,imx28-dcp']
['fsl,imx6ull-dcp', 'fsl,imx28-dcp'] is too long

So add them to the devicetree binding.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230530100843.15072-2-stefan.wahren@i2se.comSigned-off-by: Rob Herring <robh@kernel.org>