MIPS: perf: Add hardware perf events support for Loongson-3

This patch enable hardware performance counter support for Loongson-3's
perf events.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/9618/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>