CONFIG_MACH_XILFPGA is not available for thedefaultarchitecture x86.
Result is shown for architecture mips
default or selected kernelversion does not have config value CONFIG_MACH_XILFPGA.
Result is shown for kernelversion 6.7-rc3

MIPSfpga Xilinx based boards

configname: CONFIG_MACH_XILFPGA

System type
└─>MIPSfpga Xilinx based boards
In linux kernel since version 4.1 (release Date: 2015-06-21)  
This enables support for the IMG University Program MIPSfpga platform.