dt-bindings: arm: fsl: add compatible string for Kobo Aura 2

This adds a compatible string for the Kobo Aura 2 eBook reader.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>