ARM: kirkwood: enable CONFIG_DM_USB on db-88f6281-bp

Switch to the driver model for USB on the db-88f6281-bp board.
CONFIG_BLK can't be enabled yet because mvebu_mmc.c needs converting.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>