add ITM (SWO) tracing support to Ambiq Apollo3 targets

targets/TARGET_Ambiq_Micro/TARGET_Apollo3/device/itm_api.c

@@ -0,0 +1,37 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2017 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#if DEVICE_ITM
+
+#include "hal/itm_api.h"
+#include "cmsis.h"
+#include "am_bsp.h"
+
+#include <stdbool.h>
+
+/* SWO frequency: 1000 kHz */
+
+// As SWO has to be accessible everywhere, including ISRs, we can't easily
+// communicate the dependency on clocks etc. to other components - so this
+// function checks that things appear to be set up, and if not re-configures
+// everything
+void itm_init(void)
+{
+    am_bsp_itm_printf_enable();
+}
+
+#endif