[cli] support 1 or more mcast addresses for ipmaddr add command (#7253)

Because the 2 addresses are registered sequentially from the same
method call, the OpenThread stack will not have had the opportunity to
send out MLR.req with a single address. So this should result in
sending out one MLR.req with two addresses inside, if 2 addresses are
provided.  Without changes in the APIs.

src/cli/cli.cpp

@@ -2132,12 +2132,20 @@ exit:
 
 otError Interpreter::ProcessIpMulticastAddrAdd(Arg aArgs[])
 {
-    otError      error;
     otIp6Address address;
 
+#if OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE
+    otError error = OT_ERROR_INVALID_ARGS;
+    for (Arg *arg = &aArgs[0]; !arg->IsEmpty(); arg++)
+    {
+        SuccessOrExit(error = arg->ParseAsIp6Address(address));
+        SuccessOrExit(error = otIp6SubscribeMulticastAddress(GetInstancePtr(), &address));
+    }
+#else
+    otError error;
     SuccessOrExit(error = aArgs[0].ParseAsIp6Address(address));
     error = otIp6SubscribeMulticastAddress(GetInstancePtr(), &address);
-
+#endif
 exit:
     return error;
 }