MIPS: Loongson64: Add Mail_Send support for 3A4000+ CPU

Loongson 3A4000+ CPU has per-core Mail_Send register to send mail,
there is no need to maintain register address of each core and node,
just simply specify cpu number.

Signed-off-by: Lu Zeng <zenglu@loongson.cn>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>