pm: Fix cpus active count

Only set a cpu as active (on pm subsystem) when the cpu is effectively
initialized. We cannot assume on pm subsystem that all cpus were
initialized since when the option CONFIG_SMP_BOOT_DELAY is used cpus are
initialized on demand by the application.

Note that once cpus are properly initialized the subystem is able to track
their status.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>