MIPS: Loongson64: Set IPI_Enable register per core by itself

In the current code, for example, core 1 sets Core[0, 1, 2, 3]_IPI_Enalbe
register and core 2, 3 do the same thing on the 1-way Loongson64 platform,
this is not necessary. Set IPI_Enable register per core by itself to avoid
duplicate operations and make the logic more clear.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>