CONFIG_ARCH_U8500 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

ST-Ericsson U8500 Series

configname: CONFIG_ARCH_U8500

Linux Kernel Configuration
└─>ST-Ericsson U8500 Series
In linux kernel since version 2.6.33 (release Date: 2010-02-24)  
Support for ST-Ericsson's Ux500 architecture


depends
CONFIG_MMU

is selected by
CONFIG_CLKSRC_NOMADIK_MTU