tests/intel_adsp: MP core power fixups for older cAVS platforms

On cAVS before 2.5, core power was controlled by the host.  Add a
command to the cavstool.py script to allow us to do that under test
command so we can exercise multiprocessor startup/shutdown outside of
SOF.

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