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

The Marvell evaluation board (DB) for the Armada XP SoC has 6
physicals 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>