MMC/SD/SDIO over SPI

modulename: mmc_spi.ko

and/or

of_mmc_spi.ko

configname: CONFIG_MMC_SPI

Linux Kernel Configuration
└─>Device Drivers
└─>MMC/SD/SDIO card support
└─>MMC/SD/SDIO over SPI
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Some systems access MMC/SD/SDIO cards using a SPI controller
instead of using a "native" MMC/SD/SDIO controller. This has a
disadvantage of being relatively high overhead, but a compensating
advantage of working on many systems without dedicated MMC/SD/SDIO
controllers.

If unsure, or if your system has no SPI master driver, say N.

source code: