pm: policy: Selectively compile on "zephyr,power-state" compatible

Coverity is complaining:

      CID 316017:  Control flow issues  (NO_EFFECT)
  >>> This less-than-zero comparison of an unsigned value is never true.
  148      for (size_t i = 0; i < ARRAY_SIZE(substate_lock_t); i++) {

This is a false positive but we can still optimize the code and making
coverity happy by simply avoiding compiling the offending code when no
power states are defined into the DT.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>