smp: Create a new function to bringup nonboot cpus online

This is the last direct user of cpu_up() before we can hide it now as
internal implementation detail of the cpu subsystem.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Thomas Gleixner <tglx@linutronix.de>
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CC: Nicholas Piggin <npiggin@gmail.com>
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CC: Ingo Molnar <mingo@kernel.org>
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CC: Zhenzhong Duan <zhenzhong.duan@oracle.com>
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