MIPS: ath79: allow to specify bus number in PCI IRQ maps

This is needed for multiple PCI bus support.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4913/


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