MIPS: Oprofile: Fixup of loongson2_exit()

MIPS: Oprofile: Fixup of loongson2_exit()

When exiting from loongson2_exit(), we need to reset the counter
register too, this patch adds a function reset_counters() to do it, by
the way, this function will be shared by Perf.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/1199/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>