CONFIG_ARCH_AT91 is not available for the selected architecture 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 Atmel
AT91RM9200 and AT91SAM9* processors.