The AGESA source sets up the PLLs for the various function blocks on the chip.

I just want to try to warn you not to overclock your CPU. Or, use the vendor BIOS to do it. Overclocking relies on knowing the motherboard layout, timing, and thermal characteristics to a degree of detail that just isn't possible in coreboot. (And you will also want to be careful of power stability, PLL quirks, and motherboard functions that will be bricked if you drive them wrong.)

In other words, there's a reason coreboot doesn't support overclocking. Not to say a vendor couldn't add support for it.

David