The interrupt enable bit for the performance counters is in the Control
    Register $24, not in the counter register.
    loongson2_perfcount_handler(), we need to use
Reported-by: NXu Hengyang <hengyang@mail.ustc.edu.cn>
Signed-off-by: NWu Zhangjin <wuzhangjin@gmail.com>
    Cc: linux-mips@linux-mips.org
    Patchwork: http://patchwork.linux-mips.org/patch/1198/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

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