soc/intel_adsp: Clean up MP startup

The multiprocessor entry code here had some bits that look to have
been copied from esp32, including a clumsy stack switch that's needed
there.  But it wasn't actually switching the stack at all, which on
this device is pointed at the top of HP-SRAM and can stay there until
the second CPU swaps away into a real thread (this will need to change
once we support >2 CPUS though).

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