MIPS: Introduce arch/mips/Kbuild

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
To: linux-mips <linux-mips@linux-mips.org>
To: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1300/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>