CONFIG_SOC_AR913X is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SOC_AR913X (not configurable)

configname: CONFIG_SOC_AR913X

Linux Kernel Configuration
└─>SOC_AR913X
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
depends
CONFIG_ATH79

is selected by
CONFIG_USB_EHCI_ATH79

is depended by
CONFIG_ATH79_MACH_AP81