ARM: S3C24XX: Add documentation for arch/arm/plat-s3c24xx/include/plat/mci.h

Add documentation for the platform data structure in the SD/MMC driver
header file arch/arm/plat-s3c24xx/include/plat/mci.h.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>