mmc: sdhci: clarify DDR timing mode between SD-UHS and eMMC

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

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>