I don't know much and can't speak for coreboot, but have you looked at free bootloaders for ARM ? I thought coreboot was centered in x86 because it was a popular arquitecure and it usually lacks free firmware, while ARM or MIPS are most often used with free bootloaders already.
Might it be better to add PCIe (if not present) to one of those, say U-boot or something ?
What would be the advantages of porting coreboot to ARM ?
All of the free bootloaders now are focus on embedded systems, but with the development of ARM architecture, it will not only be used on embedded systems but also servers and low power consumption PCs. So we need a full-function bootloader for them.