arm: mvebu: enable two USB interfaces on the Armada XP GP board

The Armada XP GP board has two USB slots: one on the front side and
one on the back side. This commit enables the two USB host controllers
that correspond to those wo USB slots.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>