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

The Armada XP SoCs have multiple PCIe interfaces. The MV78230 has 2
PCIe units (one 4x or quad 1x, the other 1x only), the MV78260 has 3
PCIe units (two 4x or quad 1x and one 4x/1x), the MV78460 has 4 PCIe
units (two 4x or quad 1x and two 4x/1x). We therefore add the
necessary Device Tree informations to make those PCIe interfaces
usable.

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