ARM: dts: imx6ul: fix csi node compatible

"fsl,imx6ul-csi" was never listed as compatible to "fsl,imx7-csi", neither
in yaml bindings, nor previous txt binding. Remove the imx7 part. Fixes
the dt schema check warning:
csi@21c4000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx6ul-csi', 'fsl,imx7-csi'] is too long
Additional items are not allowed ('fsl,imx7-csi' was unexpected)
'fsl,imx8mm-csi' was expected

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>