NXP tested imx8mn-ddr4 with firmware version 201810 only. Use this
version for all imx8mn targets with DRAM DDR4.
Fixes: 93c4c0e4dd ("arm: dts: imx8mn-u-boot: Create common imx8mn-u-boot.dtsi")
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Reviewed-by: Fabio Estevam <festevam@denx.de>