Compilation warning fixes on 32b platfrom with IAR

Fix compilation warnings with IAR toolchain, on 32 bit platform.
Reported by rahmanih in #683

This is based on work by Ron Eldor in PR #750.
diff --git a/ChangeLog b/ChangeLog
index 93d017c..838238b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,7 +3,8 @@
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
 Bugfix
-
+   * Fix compilation warnings with IAR toolchain, on 32 bit platform.
+     Reported by rahmanih in #683
    * Fix braces in mbedtls_memory_buffer_alloc_status(). Found by sbranden, #552.
    * Added the macro MBEDTLS_X509_MAX_FILE_PATH_LEN that enables the user to
      configure the maximum length of a file path that can be buffered when