MIPS: DTS: Loongson64: Add PCI Controller Node

Add PCI Host controller node for Loongson64 with RS780E PCH dts.
Note that PCI interrupts are probed via legacy way, as different
machine have different interrupt arrangement, we can't cover all
of them in dt.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>