CONFIG_ARCH_AT91 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Atmel AT91

configname: CONFIG_ARCH_AT91

ARM system type
└─>Atmel AT91
In linux kernel since version 2.6.18 (release Date: 2006-09-19)  
This enables support for systems based on the Atmel AT91RM9200,
AT91SAM9 and AT91CAP9 processors.


is selected by
CONFIG_BLK_DEV_IDE_AT91