mmc: s3c: Add SD driver

Implement SD driver for the S3C24xx family. This implementation
is currently only capable of using the PIO transfers, DMA is not
supported.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>