To access the uboot prompt you’ll need to connect an external USB to Serial bridge to the 3 pins next to the USB-C header, but you don’t need to do that.
You can apply the overlay in Linux.
The file you want to change is here: /boot/firmware/extlinux/extlinux.conf
You’ll want to modify the file and add the following line to load the OV5640 DTBO:
fdtoverlays /overlays/k3-am625-lc-csi2-ov5640.dtbo
Then you can reboot: sudo reboot
Camera should now work, you can use mplayer to test.
sudo apt-get install -y mplayer
mplayer tv: // -tv driver=v4l2:device=/dev/video0:width=640:height=480:fps=30:outfmt=yuy2