The commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=58eae1416b804d900014d84feadda7195007cc30, added in 4.15-rc9, adds a dependency on PCI_DRIVERS_LEGACY to SSB_PCIHOST_POSSIBLE. However, it appears that PCI_DRIVERS_LEGACY is only defined for MIPS. As such, it no longer seems possible to compile the SSB_PCIHOST driver (or anything that depends on it) on at least the x86_64 architecture, but quite possibly others as well. Reverting the patch allows the drivers to be built successfully.
I am unable to reproduce. Would you please attach the .config file you are using?
No .config file is necessary. Just do a "make menuconfig" on a fresh checkout on an x86_64 system and note that under Drivers>Sonics Silicon Backplane there is no option for PCI support. You can also do a search from menuconfig for the PCI_DRIVERS_LEGACY symbol and see there is no place to enable that option, which is the reason why the PCI option for SSB does not appear.
That makes much more sense. I'll look into that more tomorrow or the day after.
Looking into it more, I would need a more thorough understanding of the SSB/PCI stuff. I am not the right person to address this.
Fixed by this commit: commit a9e6d44ddeccd3522670e641f1ed9b068e746ff7 Author: Sven Joachim <svenjoac@gmx.de> Date: Fri Jan 26 10:38:01 2018 +0100 ssb: Do not disable PCI host on non-Mips