MIPS: ath79: add PCI IRQ handling code for AR724X SoCs

The PCI Host Controller of the AR724x SoC has a
built-in IRQ controller. The current code does
not supports that, so the IRQ lines wired to this
controller are not usable. This leads to failed
'request_irq' calls:

  ath9k 0000:00:00.0: request_irq failed
  ath9k: probe of 0000:00:00.0 failed with error -89

This patch adds support for the IRQ controller
in order to make PCI IRQs work.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc:  linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3496/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>