MIPS: SGI-IP27: use generic PCI driver

Converted bridge code to a platform driver using the PCI generic driver
framework and use adding platform devices during xtalk scan. This allows
easier sharing bridge driver for other SGI platforms like IP30 (Octane) and
IP35 (Origin 3k, Fuel, Tezro).

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
[paul.burton@mips.com:
  - Leave __phys_to_dma(), __dma_to_phys() & pcibus_to_node() in
    arch/mips/pci/pci-ip27.c since the motivation for moving them
    disappeared when the driver stopped being moved to drivers/pci.]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org