MIPS: Better than nothing implementation of PCI mmap to fix X.

Certain X11 servers such as the SIS server will only work if PCI mmap is
implemented.  This patch implements PCI mmap but to be on the same side
so close to a release it only supports uncached mappings so performance
will not be optimal for some uses such as framebuffers.

Thanks to Zhang Le <r0bertz@gentoo.org> for the original report and
testing.

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