arm: kirkwood: convert MPL CEC4 to use DT for the PCIe interface

Now that the PCIe mvebu driver is usable on Kirkwood, use it instead
of the legacy PCIe code, since it allows to describe the PCIe
interfaces in the Device Tree.

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