[MIPS] Iomap implementation.

This implementation has support for the concept of one separate ioport
address space by PCI domain.  A pointer to the virtual address where
the port space of a domain has been mapped has been added to struct
pci_controller and systems should be fixed to fill in this value. For
single domain systems this will be the same value as passed to
set_io_port_base().

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>