PCI: mvebu: Move port parsing and resource claiming to separate function

Move the PCIe port parsing and resource claiming to a separate function in
preparation to add proper cleanup of claimed resources.

Tested-by: Willy Tarreau <w@1wt.eu> (Iomega iConnect Kirkwood, MiraBox Armada 370)
Tested-by: Andrew Lunn <andrew@lunn.ch> (D-Link DIR664 Kirkwood)
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> (Armada XP GP)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>