mmc: remove s3cmci driver

The s3c24xx platform is gone, so this driver can be removed as well.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>