MIPS: Loongson 3: Add CPU hotplug support

Tips of Loongson's CPU hotplug:
1, To fully shutdown a core in Loongson 3, the target core should go to
   CKSEG1 and flush all L1 cache entries at first. Then, another core
   (usually Core 0) can safely disable the clock of the target core. So
   play_dead() call loongson3_play_dead() via CKSEG1 (both uncached and
   unmmaped).
2, The default clocksource of Loongson is MIPS. Since clock source is a
   global device, timekeeping need the CP0' Count registers of each core
   be synchronous. Thus, when a core is up, we use a SMP_ASK_C0COUNT IPI
   to ask Core-0's Count.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Hongliang Tao <taohl@lemote.com>
Signed-off-by: Hua Yan <yanh@lemote.com>
Tested-by: Alex Smith <alex.smith@imgtec.com>
Reviewed-by: Alex Smith <alex.smith@imgtec.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
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/6639
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

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