MIPS: Loongson-3: Add oprofile support

Loongson-3 has two groups of performance counters, they are 4 sub-
registers of CP0's REG25. This patch add oprofile support.

REG25, sel 0: Perf Control of group 0;
REG25, sel 1: Perf Counter of group 0;
REG25, sel 2: Perf Control of group 1;
REG25, sel 3: Perf Counter of group 1.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: John Crispin <john@phrozen.org>
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/8328/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>