MIPS: ath79: add support for the PCI host controller of the AR71XX SoCs

The Atheros AR71XX SoCs have a built-in PCI Host Controller.
This patch adds a driver for that, and modifies the relevant
files in order to allow to register the PCI controller from
board specific setup.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3498/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>