arm: kirkwood: convert ZyXEL NSA310 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.

Since it was the only device left that prevented this platform to use
the Device Tree only, we remove the board-nsa310.c file and the
related Kconfig/Makefile bits.

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