arm: mvebu: db-88f6820-gp: Add USB/EHCI support

This patch enabled the USB/EHCI support for the Marvell
DB-88F6820-GP eval board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>