mmc: core: mmc DDR mode should not depend on UHS_DDR50

The MMC_CAP_UHS_DDR50 must work on 1.8v.
However, the eMMC DDR mode can work on either 1.8v or 3.3v and
should not depend on UHS_DDR50.
So get rid of this limitation to let controller without 1.8v
signal voltage support can also work for eMMC DDR mode if it claims.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>