MIPS: Loongson64: Add generic dts

Add generic device dts for Loongson-3 devices.
They are currently almost identical but will be different later.
Some PCH devices like PCI Host Bridge is still enabled by platform
code for now.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>