powerpc/4xx: PCIe driver now detects if a port is disabled via the dev-tree

This patch add a check to the PPC4xx PCIe driver to detect if the port
is disabled via the device-tree. This is needed for the AMCC Canyonlands
board which has an option to either select 2 PCIe ports or 1 PCIe port
and one SATA port. The SATA port and the 1st PCIe port pins are multiplexed
so we can't start both drivers.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>