MIPS: ath79: add IRQ handling code for the QCA955X SoCs

MIPS: ath79: add IRQ handling code for the QCA955X SoCs

The IRQ routing in the QCA955x SoCs is slightly
different from the routing implemented in the
already supported SoCs.

Cc: Rodriguez, Luis <rodrigue@qca.qualcomm.com>
Cc: Giori, Kathy <kgiori@qca.qualcomm.com>
Cc: QCA Linux Team <qca-linux-team@qca.qualcomm.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4955/
Signed-off-by: John Crispin <blogic@openwrt.org>