[MIPS] Fix modpost warnings by making start_secondary __cpuinit

WARNING: arch/mips/kernel/built-in.o(.text+0x9a58): Section mismatch: reference to .init.text:cpu_report (between 'start_secondary' and 'smp_prepare_boot_cpu')
WARNING: arch/mips/kernel/built-in.o(.text+0x9a60): Section mismatch: reference to .init.text:per_cpu_trap_init (between 'start_secondary' and 'smp_prepare_boot_cpu')
WARNING: arch/mips/kernel/built-in.o(.text+0x9adc): Section mismatch: reference to .init.text:cpu_probe (between 'start_secondary' and 'smp_prepare_boot_cpu')
mipsel-linux-objcopy -S -O srec --remove-section=.reginfo --remove-section=.mdebug --remove-section=.comment --remove-section=.note --remove-section=.pdr --remove-section=.options --remove-section=.MIPS.options vmlinux arch/mips/boot/vmlinux.srec

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