Locations of SEEPROM and BMC SPI Flash
I can find this reference on IBM's site, https://developer.ibm.com/technologies/linux/articles/protect-system-firmware-openpower
The SBE has several stages, with the first part stored in processor one-time programmable read-only memory (OTPROM) and another part stored in the SEEPROM module located on the POWER processor module.
which suggests that SEEPROM is actually a tiny chip outside the POWER9?/POWER8? die, but I am not positive.
Also, this wiki page mentions "BMC SPI Flash", but I am unsure if this is a separate physical flash chip from PNOR?
It is practically guaranteed that the CPU SEEPROM is a separate die to the main POWER9 die, probably attached via I2C. Consider this photo of a POWER8 module without the heatspreader, probably some of those additional chips are SEEPROMs.