dt-bindings: fsl: add kamstrup flex concentrator to schema

Add Kamstrup OMNIA Flex Concentrator compatibles to the schema
so we can make use of them for the validation.

Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>