mmc: dw_mmc: Fix DDR mode support.

Host driver can't get a hint of DDR mode through ios->ddr flag anymore.
ios->timing is currently used to inform DDR mode as a substitute.
And capability of MMC_CAP_MMC_HIGHSPEED is added for DDR support.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>