MIPS: pci: convert lantiq driver to OF

Implement support for OF inside the lantiq PCI driver. The patch also splits
pcibios_plat_dev_init and pcibios_map_irq out into their own file to accomodate
coexistance with the upcoming pcie driver.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3806/


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