MIPS: ath79: separate common PCI code

MIPS: ath79: separate common PCI code

The 'pcibios_map_irq' and 'pcibios_plat_dev_init'
are common functions and only instance one of them
can be present in a single kernel.

Currently these functions can be built only if the
CONFIG_SOC_AR724X option is selected. However the
ath79 platform contain support for the AR71XX SoCs,.
The AR71XX SoCs have a differnet PCI controller,
and those will require a different code.

Move the common PCI code into a separeate file in
order to be able to use that with other SoCs as
well.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: René Bolldorf <xsecute@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3485/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>