[coreboot] how to add some constant bytes into .rom file in coreboot v2 and v3?

To support VIA's ROMSIP I need to add the bellow constant into .rom
file.
How should I do to achieve this?
The data I need add include:
At ffffffd0, a 4byte pointer (pointto a address X (e.g. 0xffffff70))
At X, 32byte ROMSIP data

Thank you!

-jasonzhao