PCI: Add Loongson PCI Controller support

This controller can be found on Loongson-2K SoC, Loongson-3
systems with RS780E/LS7A PCH.

The RS780E part of code was previously located at
arch/mips/pci/ops-loongson3.c and now it can use generic PCI
driver implementation.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>