kernel: smp: introduce k_smp_cpu_resume

This provides a path to resume a previously suspended CPU.
This differs from k_smp_cpu_start() where per-CPU kernel
structs are not initialized such that execution context
can be saved during suspend and restored during resume.
Though the actual context saving and restoring are
platform specific.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>