CONFIG_ARCH_AT91X40 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

AT91x40

modulename: at91x40.ko

configname: CONFIG_ARCH_AT91X40

Atmel AT91 Processor
└─>AT91x40
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
source code:
/arch/arm/mach-at91/

is selected by
CONFIG_BLK_DEV_IDE_AT91