pci: pci_mvebu: Disable config access to PCI host bridge ports

This patch changes the PCI config routines in the Armada XP / 38x driver
to not allow access to the PCIe root ports.

While updating the Armada XP based theadorable to the latest mainline
and testing it with the DM PCI driver I noticed, that the PCI root
bridge was being configured incorrectly. Resulting in the PCIe Intel
WiFi was not working correctly in Linux. With this patch applied, all
PCIe devices work without any issues in Linux again.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Mario Six <mario.six@gdsys.cc>