POWER8 is now among the architectures with upstream Coreboot support. There's also the qemu-power8 emulated mainboard as part of this initial commit. POWER8 architecture support goes alongside ARM, ARM64 (ARMv8), MIPS, RISC-V, and x86/x86_64 hardware support present in mainline Coreboot.
More details on the initial POWER8 Coreboot support can be found via this Git commit. POWER8 support in Coreboot (and then Libreboot) will be important if the Talos Secure Workstation is to pan out; earlier this week the preliminary specs were revised for this POWER8 system.