pci: pci_mvebu: Define an IO region as well

Configure an IO region and window for PNP identical to how MEM region is
set up. Linux does this only if the DT defines a pcie-io-aperture
property for the SOC, but since all supported boards do this should not
be needed.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Stefan Roese <sr@denx.de>