mmc: omap_hsmmc: Enable DDR mode support

In order to enable DDR mode, Dual Data Rate mode bit has to be set in
MMCHS_CON register. Set it here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>