added naieve way of sleeping seems to work at first glance

targets/TARGET_Ambiq_Micro/TARGET_Apollo3/device/sleep.c

@@ -0,0 +1,28 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2018 Arm Limited and affiliates.
+ * 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.
+ */
+#include "sleep_api.h"
+#include "am_hal_sysctrl.h"
+
+void hal_sleep(void)
+{
+  am_hal_sysctrl_sleep(AM_HAL_SYSCTRL_SLEEP_NORMAL);
+}
+
+void hal_deepsleep(void)
+{
+  am_hal_sysctrl_sleep(AM_HAL_SYSCTRL_SLEEP_DEEP);
+}

targets/targets.json

@@ -7185,7 +7185,8 @@
             "STDIO_MESSAGES",
             "FLASH",
             "SPI",
-            "I2C"
+            "I2C",
+            "SLEEP"
         ],
         "components": [
             "FLASHIAP"