MIPS: Loongson: Allow booting from any core

By offering Logical->Physical core id mapping, so as to reserve some
physical cores via mask. This allow booting from any core when core-0
has problems. Since the maximun cores supported by Loongson-3 is 16,
32-bit cpu_startup_core_id can be split to 16-bit cpu_startup_core_id
and 16-bit reserved_cores_mask for compatibility.

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/8323/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>