MIPS: Simplify the weak annotation with __weak

Found by

$ find arch/mips/ -name "*.c" | xargs -i grep -H weak {} | grep -v __weak

[Ralf: Made this bulletproof by including <linux/compiler.h>]

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/874/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>