Add changelog for 5.3.1.

Change-Id: I197a9c6a0fc623bdb3eafaa5fe970cb6a44546b4
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Merge remote-tracking branch 'origin/5.3.1' into 5.3

Change-Id: I6abb3fad4e03323919a9ee4dad62082784ac6160