dt-bindings: arm: Add Beacon EmbeddedWorks i.MX8M Plus kit

Add DT compatible string for a Beacon EmbeddedWorks development
kit based on the i.MX8M Plus from NXP.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>