soc/intel_adsp: Clean up IPM compiler guards

The CAVS_IDC_IPM driver happens to be used only on non-2.5 hardware,
but it's best to be clear in the conditional compilation when we're
talking about hardware-dependencies and when we mean software
configuration.  This was mixed up in a few spots.

Also fix a warning that creeps in on non-default drivers choices about
an undeclared ipm function.

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