soc/intel_adsp: Rework core start/restart API

Remove the soc_relaunch_cpu() API.  There's now nothing that this does
that isn't already done by the kernel's own z_smp_start_cpu() API.  We
don't need a SOC-level API for this anymore.

Rename soc_halt_cpu() to soc_adsp_halt_cpu() to clarify the domain,
simplify by removing the synchronization (there's nothing to
synchronize!).  Also move its declaration and docs into the soc.h
header.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>