soc: intel_adsp/cavs_v25: Add CPU halt and relaunch APIs

Add a SOC API to allow for application control over deep idle power
states.  Note that the hardware idle entry happens out of the WAITI
instruction, so the application has to be responsibile for ensuring
the CPU to be halted actually reaches idle deterministically.  Lots of
warnings in the docs to this effect.

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