CONFIG_SOC_AR724X is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SOC_AR724X (not configurable)

configname: CONFIG_SOC_AR724X

Linux Kernel Configuration
└─>SOC_AR724X
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  

depends
CONFIG_ATH79


is depended by
CONFIG_ATH79_MACH_UBNT_XM