drivers/rpmsg_rtc: don't rpmsg send for some core

without rpmsg rtc client driver

Avoid sending rpmsg messages to cores that don't have RTC client driver.
Check endpoint availability before attempting to send messages.

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

drivers/timers/rpmsg_rtc.c

@@ -747,6 +747,11 @@ static void rpmsg_rtc_server_ns_bound(FAR struct rpmsg_endpoint *ept)
 
   client = container_of(ept, struct rpmsg_rtc_client_s, ept);
   server = client->ept.priv;
+
+  nxmutex_lock(&server->lock);
+  list_add_tail(&server->list, &client->node);
+  nxmutex_unlock(&server->lock);
+
   rpmsg_rtc_server_sync(server, client);
 }
 
@@ -774,10 +779,6 @@ static void rpmsg_rtc_server_created(FAR struct rpmsg_device *rdev,
       kmm_free(client);
       return;
     }
-
-  nxmutex_lock(&server->lock);
-  list_add_tail(&server->list, &client->node);
-  nxmutex_unlock(&server->lock);
 }
 #endif