ARM: imx: imx8mn-ddr4-evk: Add ethernet support

Add support for ethernet on the imx8mn-ddr4-evk.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>