arm: dts: imx8mn-u-boot: fix DDR3 only support

In case the CONFIG_IMX8M_LPDDR4 and CONFIG_IMX8M_DDR4 options are both
disabled (i. e. BSH boards), binmain fails because DDR4 bin files are
missing.

Fixes: 93c4c0e4 ("arm: dts: imx8mn-u-boot: Create common imx8mn-u-boot.dtsi")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>