Hi Bruno,
people were quite busy with the coreboot conference last week. In case you didn't find answers on your own yet:
My question is: Why flashrom says it doesn't support my motherboard if my chipset is supported on the coreboot project wiki.
Your question somehow answers itself: For coreboot, every motherboard has to be supported individually. And that's sometimes also true for flashrom. So in your case, flashrom may know how to access the flash chip but doesn't know if there are other programs / chips like the EC that may intervene. Maybe try a newer version of flashrom, yours seems outdated. In the end, you can try to force flashrom, but this could brick your machine.
Also, if you target at running coreboot on your machine, I don't think it's supported. But that's a question for the coreboot mailing list.
Nico