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 d9682a5..cf29b0a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS x.x.x branch released xxxx-xx-xx
+= mbed TLS 2.x.x released xxxx-xx-xx
 
 Features
    * Add the functions mbedtls_platform_setup() and mbedtls_platform_teardown()