Requirements for PCIe Bifurcation on SBC
It’s entirely dependent on the root complex and its capabilities. Bifurcation is a fairly recent development only showing up in non-server x86 motherboards about 1-1/2 years ago (e.g., Intel x299 / AMD x399.) Server motherboards have supported it a bit longer using switches. 

Not only that, even if the hardware supports it, the BIOS needs to support it, too.

So it’s a question for Rockchip. My guess is the RK3399 does not support it. (Followup: it doesn't.)

That said, if you’re just doing SATA then you don’t really need bifurcation, the PCIe-SATA bridge takes can take care of that (4-port PCIe - SATA adapters are common and cheap.) You would only need to bifurcate if you wanted direct tie to PCIe / NVMe. Or maybe I’m misunderstanding your question?