CONFIG_SOC_AR71XX is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SOC_AR71XX (not configurable)

configname: CONFIG_SOC_AR71XX

Linux Kernel Configuration
└─>SOC_AR71XX
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  

depends
CONFIG_ATH79

is selected by
CONFIG_USB_OHCI_ATH79

is depended by
CONFIG_ATH79_MACH_PB44