arm: mvebu: PCIe Device Tree informations for Armada XP GP

The Marvell Armada XP GP board has 3 physical full-size PCIe slots, so
we enable the corresponding PCIe interfaces in the Device Tree.

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