The memory controller is probably configured by the Intel FSP binary blob so you can only tweak what it allows you to.

Firmware is specific to each motherboard but if you know what you're doing and have motherboard schematics you can modify Coreboot to support different motherboards that use already-supported chips.

Thanks for the info. How would you go about figuring out what the blob allows you to configure? Is it documented anywhere (e.g. an Intel Manual)?