mmc: support the DDR mode for eMMC

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>