Add BSD-3 license to endianness agnostic files (#417)

Co-authored-by: amgross <aryespa@gmail.com>

Author: amgross

erpc_c/port/erpc_endianness_agnostic_example.h

@@ -1,3 +1,7 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
 /*
  * This is example file for endianness agnostic communication based on byteswap.h.
  * Other approach can be done with htons(), htonl(), ntohs(), ntohl() and such.

erpc_c/port/erpc_endianness_undefined.h

@@ -1,3 +1,6 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
 
 #ifndef _ERPC_ENDIANNESS_UNDEFINED_H_
 #define _ERPC_ENDIANNESS_UNDEFINED_H_