arm64: dts: imx8mm-iot-gateway: Add initial support

Add the initial support for the CompuLab i.MX8MM IoT Gateway board,
which is based on the Compulab's UCM-iMX8M-Mini SoM.

Board URL:
https://www.compulab.com/products/iot-gateways/iot-gate-imx8-industrial-arm-iot-gateway/

SoM URL:
https://www.compulab.com/products/computer-on-modules/ucm-imx8m-mini-nxp-i-mx-8m-mini-som-system-on-module-computer/



Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>