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

Replaced UHS_DDR50 with MMC_DDR52.

CC: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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>