Hi,
forget everything I said; this was a hardware issue.
When I was first flashing coreboot, I had some problems with my SPI programmer and I was unsure if I had damaged the flash chip. So I replaced it (a MX25L3206E) with another one - a Winbond W25Q32 - that I programmed externally before soldering it on the board. I expected it to be sufficiently similar to the Macronix chip, and when flashing and booting and reprogramming from Linux worked without problems, I did no longer think about it. But obviously there are some subtle differences. I put back the MX25L3206E on the board, and now everyting, including suspend and resume, works fine.
Nevertheless, thanks for your help!
Gert