MIPS: Loongson: arch/mips/Makefile: Add missing whitespace

This patch add missing whitespace after every "+=" in the loongson
related part of arch/mips/Makefile.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: yanh@lemote.com
Cc: huhb@lemote.com
Cc: zhangfx@lemote.com
Patchwork: http://patchwork.linux-mips.org/patch/827/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>