Add fix of Mbed-TLS#1722 to ChangLog

Signed-off-by: Jeffrey Martin <Jeffrey_Martin@rapid7.com>

ChangeLog

@@ -11,6 +11,10 @@ Bugfix
      previously lead to a stack overflow on constrained targets.
    * Add `MBEDTLS_SELF_TEST` for the mbedtls_self_test functions
      in the header files, which missed the precompilation check. #971
+   * Extend register cleanup needed on MIPS assembly optimizations. Adds hi, lo
+     registers hints to the compiler for usage with the embedded MIPS assembly.
+     Reported in #1722, fix suggested by @aurel32 and submitted
+     by Jeffrey Martin, #1949.
 
 = mbed TLS 2.16.0 branch released 2018-12-21