update mbed overrides to support SVL bootloader

Author: Kyle Wenner

targets/TARGET_Ambiq_Micro/TARGET_Apollo3/device/mbed_overrides.c

@@ -0,0 +1,7 @@
+#include "am_hal_interrupt.c"
+
+// called before main
+void mbed_sdk_init()
+{
+    am_hal_interrupt_master_enable();
+}