MIPS: ath79: allow to use SoC specific PCI IRQ maps

MIPS: ath79: allow to use SoC specific PCI IRQ maps

The PCI controllers in the AR71XX and in the
AR724X SoCs are different, and both of them
uses different IRQ wiring.

The patch modifies the 'pcibios_map_irq' function
in order to allow to use different IRQ maps for
the different SoCs. The patch also adds a function,
which lets the board setup code to override the
default IRQ map.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3500/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>