bsp/tms570: Relicense to BSD-2-Clause

Change license to BSD-2-Clause according to file history and contributor
agreements.  Add Doxygen file comments.

Update #3053.
Update #3707.
Update #4982.

bsps/arm/tms570/start/init_system.c

@@ -1,6 +1,11 @@
-/** @file
+/* SPDX-License-Identifier: BSD-3-Clause */
 
-   based on Ti HalCoGen generated file
+/**
+ * @file
+ *
+ * @ingroup RTEMSBSPsARMTMS570
+ *
+ * @brief This source file contains parts of the system initialization.
  */
 
 /*