dt-bindings: arm: fsl: Add beacon,imx8mn-beacon-kit

Add beacon,imx8mn-beacon-kit to list of compatible options.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>