dt-bindings: Document Loongson PCI Host Controller

PCI host controller found on Loongson PCHs and SoCs.

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