imx: Add support for i.MX8MN Beacon EmbeddedWorks devkit.

Beacon EmbeddedWorks is releasing a devkit based on the i.MX8M
Nano SoC consisting of baseboard + SOM.

The kit is based on the same design as the Beacon dev kit with
the i.MX8M Mini.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>