imx8mn has an extra cortex-m7 core. you have few options for starting it
- use downstream imx-atf and mainline u-boot
bootaux
command - use upstream atf and poke at the registers manually
For option 1 you can follow the AN5317 app note and things should work out fine
fatload mmc 1:1 0x48000000 hello.bin
cp.b 0x48000000 0x7e0000 $filesize
bootaux 0x7e0000