arm: socfpga: Nuke useless include

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>