MIPS: Fix global namespace pollution in arch/mips/kernel/smp-up.c

The following symbols in arch/mips/kernel/smp-up.c are needlessly
defined global:

up_send_ipi_single()
up_init_secondary()
up_smp_finish()
up_cpus_done()
up_boot_secondary()
up_smp_setup()
up_prepare_cpus()

This patch makes the symbols static.

Build-tested using malta_defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>