Goodcell EV charger: add optional dp 120.

From PR #2106, this appears to be a charging session duration, but is a string,
so not directly useful as a sensor.

custom_components/tuya_local/devices/goodcell_ev_charger.yaml

@@ -262,6 +262,10 @@ secondary_entities:
         range:
           min: 0
           max: 15
+      - id: 120
+        type: string
+        optional: true
+        name: session_duration
   - entity: select
     name: Toggle charging
     icon: "mdi:ev-plug-type2"