imx8mn_ddr4_evk: Allow booting the kernel by default

Currently it is not possible to boot Linux from the SD card
by default.

Make the necessary adjustments to allow it.

Reported-by: Peter Bergin <peter@berginkonsult.se>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Andrey Zhizhikin <andrey.z@gmail.com> # imx8mn_ddr4_evk
Reviewed-by: Peng Fan <peng.fan@nxp.com>