MIPS: Loongson64: Make some functions static in smp.c

Make some functions static to fix the following sparse warnings:

  arch/mips/loongson64/smp.c:54:5: sparse: sparse: symbol 'ipi_read_clear' was not declared. Should it be static?
  arch/mips/loongson64/smp.c:55:6: sparse: sparse: symbol 'ipi_write_action' was not declared. Should it be static?
  arch/mips/loongson64/smp.c:56:6: sparse: sparse: symbol 'ipi_write_enable' was not declared. Should it be static?
  arch/mips/loongson64/smp.c:57:6: sparse: sparse: symbol 'ipi_clear_buf' was not declared. Should it be static?
  arch/mips/loongson64/smp.c:58:6: sparse: sparse: symbol 'ipi_write_buf' was not declared. Should it be static?

Fixes: fed4955f ("MIPS: Loongson64: Add Mail_Send support for 3A4000+ CPU")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>