Create itm.md

Documentation for the new ITM HAL API: ARMmbed/mbed-os#5956

Author: Marcus Chang

docs/reference/contributing/target/itm.md

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+### Instrumented Trace Macrocell
+
+For targets with Arm CoreSight (e.g. Cortex-M3 and Cortex-M4) the Instrumented Trace Macrocell provides a lightweight, non-intrusive way for collecting debug trace output. 
+
+#### Assumptions
+
+* The target supports Arm CoreSight.
+* The target has SWO connected either to a compatible interface chip or exposed as a debug pin.
+
+##### Defined behavior
+
+* Targets must implement the function `void itm_init(void)` and add `ITM` to the `device_has` section in `target.json`.
+* When `void itm_init(void)` is called, the debug clock for the ITM must be initialized and the SWO pin configured for debug output.
+* `void itm_init(void)` should only have to modify the clock pre-scaler in the generic register `TPI->ACPR`. The generic ITM registers will be initialized by the helper function `mbed_itm_init`. 
+* `void itm_init(void)` will only be called once during startup and doesn't have to be protected for multiple calls.
+
+##### Undefined behavior
+
+* The debug clock frequency is left undefined since the most optimal frequency will vary from target to target. It is up to each target's owner to pick a frequency that (a) doesn't interfere with normal operation and (b) is supported by the owner's preferred debug monitor. 
+
+#### Testing
+
+The `SerialWireOutput` class can be used for sending `stdout` to the SWO stimulus port on the ITM by including this override function:
+
+```
+#include "SerialWireOutput.h"
+
+FileHandle* mbed::mbed_override_console(int fd) {
+    static SerialWireOutput swo;
+    return &swo;
+}
+```
+
+The function can be placed in any C++ compilation unit, including `main.cpp`.