imx8mn_var_som: Don't generate images including DDR4 firmware files

Binary images spl/u-boot-spl-ddr.bin and flash.bin need propietary DDR4 firmware files, which are provided by imx-firmware-ddr4 package from non-free component.

As u-boot belongs to target component, it can't have a build dependency in non-free, so these DDR4 firmware files won't be available during u-boot build.

Let's drop these images and only provide the regular u-boot binaries (without DDR4 firmware) for this board. External scripts can then pull the firmware files and build the final image.