soc/intel_adsp: Refactor mp code to separate hardware from OS details

Clean up soc_mp.c a bit.  Put all cAVS register use in functions
dedicated to hardware details (e.g. "soc_start_core()"), leave the
Zephyr OS tracking (e.g. the CPU start record, the active cores array,
etc...) in generic code.

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