cavs: unify defconfigs for v15, v18, v20 and v25

Unify default configurations to support both SMP and UP:

1. make SMP default, although it's currently disabled in prj.conf
2. use CAVS timer by default in both UP and SMP configurations
3. make MP_NUM_CPUS, IPM and IPM_CAVS_IDC depend on SMP

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>