arm64: dts: imx8mq: fix dtschema warning for imx7-csi

According to dtschema for the csi bridge, compatible is an enum and
only one must be used. Fixing this removes the following warning:

compatible: 'oneOf' conditional failed, one must be fixed

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>