MIPS: pci-ar724x: convert into a platform driver

The patch converts the pci-ar724x driver into a
platform driver. This makes it possible to register
the PCI controller as a plain platform device.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4905/


Signed-off-by: John Crispin <blogic@openwrt.org>