intel_adsp/cavs: power: Fix INTLEVEL value

In pm_state_set we can't just call k_cpu_idle() because
this will clear out PS.INTLEVEL. Use k_cpu_atomic_idle instead
since Zephyr's expect interruptions to be locked after pm_state_set.

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