ARM: cns3xxx: Add support for PCI Express ports

This patch adds PCIe support for CNS3xxx-based boards. The support
was tested with a directly attached SKY2 NIC, and EHCI USB controller
behind the PLX PEX8112 P2P bridge (to make sure that type1 cfg cycles
work as expected).

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>