mmc: sh_mmcif: Add support for eMMC Dual Data Rate

Some MMCIF implementations support the Dual Data Rate. With this patch,
platforms can set the MMC_CAP_UHS_DDR50 capability flag in MMCIF platform
data. This will let the MMC core to actually use the DDR mode.

Signed-off-by: Teppei Kamijou <teppei.kamijou.yb@renesas.com>
Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>