power: Fix dummy policy states info

../../../../../include/power/power_state.h:280:41: warning: missing
braces around initializer [-Wmissing-braces]
  280 | #define PM_STATE_DT_ITEMS_LIST(node_id) {           \
      |                                         ^

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