MIPS: Loongson: Split common loongson source code out

To share common loongson source code between all of the loongson-based
machines. there is a need to split it out of the fuloong-2e/ directory.
at the same time, other according tuning is needed. the machine-specific
parts are defined as macros in relative header file, pci.h, mem.h,
machine.h.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>