CONFIG_ARCH_IXP4XX is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

IXP4xx-based

configname: CONFIG_ARCH_IXP4XX

ARM system type
└─>IXP4xx-based
In linux kernel since version 2.6.12  
Support for Intel's IXP4XX (XScale) family of processors.


depends
CONFIG_MMU