Remove mutexes from ECP hardware acceleration

Protecting the ECP hardware acceleratior with mutexes is inconsistent with the
philosophy of the library. Pre-existing hardware accelerator interfaces
leave concurrency support to the underlying platform.

Fixes #863
diff --git a/ChangeLog b/ChangeLog
index 2f0116b..29acca9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.x.x released xxxx-xx-xx
+
+Changes
+   * Removed mutexes from ECP hardware accelerator code. Now all hardware
+     accelerator code in the library leaves concurrency handling to the
+     platform. Reported by Steven Cooreman. #863
+
 = mbed TLS 2.5.1 released 2017-06-21
 
 Security