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

ARM7 AT91X40

modulename: at91x40.ko

configname: CONFIG_ARCH_AT91X40

Core type
└─>ARM7 AT91X40
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Select this if you are using one of Atmel's AT91X40 SoC.

source code:
/arch/arm/mach-at91/
selects
CONFIG_CPU_ARM7TDMI
CONFIG_ARCH_USES_GETTIMEOFFSET
CONFIG_MULTI_IRQ_HANDLER
CONFIG_SPARSE_IRQ

depends
NOT CONFIG_MMU

is selected by
CONFIG_RTC_DRV_AT91SAM9