arm: dts: imx8mn-u-boot: use versioned ddr4 firmware

NXP tested imx8mn-ddr4 with firmware version 201810 only. Use this
version for all imx8mn targets with DRAM DDR4.

Fixes: 93c4c0e4 ("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>