soc: cavs_v25: Clean up platform config to reflect recent work

Use the built-in IDC handling and not IPM (which is limited to two
CPUs).  Declare two cpus for now, Zephyr tests are having problems
with more at the moment (that isn't a CI configuration, so we may have
work to do).

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