CONFIG_SOC_PNX8550 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SOC_PNX8550 (not configurable)

modulename: fixup-pnx8550.ko

configname: CONFIG_SOC_PNX8550

Linux/MIPS Kernel Configuration
└─>SOC_PNX8550
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/mips/pci/

is selected by
CONFIG_SERIAL_PNX8XXX