dt-bindings: mmc: add DT binding for S3C24XX MMC/SD/SDIO controller

Adds the device tree bindings description for Samsung S3C24XX
MMC/SD/SDIO controller, used as a connectivity interface with external
MMC, SD and SDIO storage mediums.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>