fix (tongou_sa1_wifi_energy_meter): add missing frequency unit

Issue #4288

custom_components/tuya_local/devices/tongou_sa1_wifi_energy_meter.yaml

@@ -79,6 +79,7 @@ entities:
         type: integer
         optional: true
         name: sensor
+        unit: Hz
         mapping:
           - scale: 100
   - entity: switch