dt-bindings: arm: fsl: add compatible string for Kobo Clara HD

This adds a compatible string for the Kobo Clara HD eBook reader.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>