default or selected kernelversion does not have config value CONFIG_MMC_AT91.
Result is shown for kernelversion 6.2.5

AT91 SD/MMC Card Interface support

modulename: at91_mci.ko

configname: CONFIG_MMC_AT91

Atmel SD/MMC Driver
└─>AT91 SD/MMC Card Interface support
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This selects the AT91 MCI controller.

If unsure, say N.

source code:
drivers/mmc/host/at91_mci.c