In that case I have misunderstood you. What do PCIe devices have to do with secure boot?

As far as I'm aware, the Platform Key is used to validate UEFI drivers' signature, so configuring Secure Boot as detailed in the Arch wiki will also provide you with the possibility to sign a UEFI driver.

I'm not sure if there are any open source UEFI drivers out there (I don't think anyone has bothered), but you could write them if you wanted to. Kind of like Windows drivers, there's just not a lot of interest in open source UEFI drivers. Secure boot isn't preventing you from writing your own drivers.