pm: Fix possible assertion when resuming

pm_system_resume() can be called from the idle thread with
interruptions unmasked. In this situation, _current_cpu will call
z_smp_cpu_mobile() that will return true and cause an assert.

In this function we don't need to check if the current context can be
preempted, we just need the information about which cpu is executing
it.

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