MIPS: Loongson: clean up the coding style

With the help of script/checkpatch.pl, i have cleaned up the coding
style.

1. remove un-needed header files and tune some comments.
2. remove some un-needed { }

add a new header file loongson.h:

3. move some common header files to loongson.h
4. move some common extern declartions to loongson.h

and this new header file is needed for future loongson2f support.

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