Added MMC_DDR52 as eMMC's DDR mode is distinguished from SD-UHS.

CC: Wei WANG <wei_wang@realsil.com.cn>
CC: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>