PCI: designware: Keep viewport fixed for IO transaction if num_viewport > 2

Most of the platforms have 3 or more viewports.  For such platforms, We do
not need to share viewports between IO and CFG.  Assign viewport 2 to IO
transactions in such cases.

Tested-by: Dong Bo <dongbo4@huawei.com>
Signed-off-by: Pratyush Anand <pratyush.anand@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>