ARM: imx: add i.MX8MN lpddr4 image cfg file

Add cfg file for i.MX8MN LPDDR4

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>