r 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>
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