mmc: host: s3cmci: allow probing from device tree

Allows configuring Samsung S3C24XX MMC/SD/SDIO controller using a device
tree.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
[Arnd: fix broken conditional expression]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>