libsdl2: upgrade to 2.0.14

This version does not support tslib any more, as can be seen by the
failed AUH run.

Originally-by: Romain Roffé <romain.roffe@blade-group.com>
Signed-off-by: Yann Dirson <yann@blade-group.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>