MIPS: Clean up arch/mips/Makefile

- Drop CLEAN_FILES assignments that is no longer required
- Add $(Q) in a few rules

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/1305/


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