The dwmmc.h include was forgotten during the migration of dwmmc
probing to DM. Since the shiny DM is in place now, remove this
relic of the past.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>