Restruture device_key

Restructure device_key to have
* include/device_key - header
* source - source file
* tests/TESTS - Greentea test

Author: LDong-Arm

drivers/device_key/README.md

@@ -42,5 +42,5 @@ To instantiate DeviceKey, you need to call its `get_instance` member function as
 Run the DeviceKey functionality test with the `mbed` command as follows:
 
 ``` 
-    ```mbed test -n features-device_key-tests-device_key-functionality```
+    ```mbed test -n drivers-device_key-tests-tests-device_key-functionality```
 ```

drivers/device_key/source/DeviceKey.h renamed to drivers/device_key/include/device_key/DeviceKey.h

@@ -23,7 +23,7 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#include "features/device_key/source/DeviceKey.h"
+#include "device_key/DeviceKey.h"
 
 #define SECURESTORE_ENABLED 1