More tolerant KNX component if gateway cant be connected (#11511)

* Issue #11432: Do not stop initializing KNX when tunelling device cant be reached

*  Issue #11432: Mark devices as unavailable if gateway cant be connected

homeassistant/components/binary_sensor/knx.py

@@ -129,6 +129,11 @@ class KNXBinarySensor(BinarySensorDevice):
         """Return the name of the KNX device."""
         return self.device.name
 
+    @property
+    def available(self):
+        """Return True if entity is available."""
+        return self.hass.data[DATA_KNX].connected
+
     @property
     def should_poll(self):
         """No polling needed within KNX."""