CONFIG_MACH_IXP4XX_OF is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Device Tree IXP4xx boards

configname: CONFIG_MACH_IXP4XX_OF

Linux Kernel Configuration
└─>Intel IXP4xx Implementation Options
└─>Device Tree IXP4xx boards
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Say 'Y' here to support Device Tree-based IXP4xx platforms.