mips: use the the PCI controller's io_map_base

commit eab90291
(mips: switch to GENERIC_PCI_IOMAP)
failed to take into account the PCI controller's
io_map_base for mapping IO BARs.
This also caused a new warning on mips.

Fix this, without re-introducing code duplication,
by setting NO_GENERIC_PCI_IOPORT_MAP
and supplying a mips-specific __pci_ioport_map.

Reported-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>