MIPS: Add driver for the built-in PCI controller of the RT3883 SoC

MIPS: Add driver for the built-in PCI controller of the RT3883 SoC

The Ralink RT3883 SoCs have a built-in PCI Host Controller
device. The patch adds a platform driver and device tree
binding documentation for that.

The patch also enables the HW_HAS_PCI config option. This
is required in order to be able to enable the PCI support.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/5758/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>