NonCopyable: Add into platform doc group.

Author: pan-

platform/NonCopyable.h

@@ -23,6 +23,13 @@
 
 namespace mbed {
 
+/** \addtogroup platform */
+/** @{*/
+/**
+ * \defgroup platform_NonCopyable NonCopyable class
+ * @{
+ */
+
 /**
  * Prevents generation of copy constructor and copy assignment operator in 
  * derived classes.
@@ -218,6 +225,10 @@ class NonCopyable {
 #endif 
 };
 
+/**@}*/
+
+/**@}*/
+
 } // namespace mbed
 
 #endif /* MBED_NONCOPYABLE_H_ */