@Suiseiseki It's needed to initialise the system. However, rockchip posted source code for at least part of the ATF: https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/16952
The one they released source for lacks scmi support so CPU/GPU reclocking unfortunately doesn't work with it. I don't know if a third party can add support for that.
I suspect they'll follow this up with a more complete submission when they get the time. From what I gather, rockchip's software team is feeling some pretty bad crunch in the past year or so.