Ok, so I think these pages might have the answer, haven’t tried it yet.
https://docs.armbian.com/User-Guide_Armbian_overlays/
and
It looks like, I would create a file called something like rk3308-usb-otg.dts in the /boot/overlay-user folder, with the contents you wrote above.
execute armbian-add-overlay rk3308-usb-otg.dts
reboot
That should be it?