arm64: dts: freescale: introduce dimonoff-gateway-evk board

The Dimonoff gateway EVK board is based on a Variscite
VAR-SOM-NANO, with a NXP MX8MN nano CPU and also based on a Symphony
mx8mn EVK.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>