arm: imx25-pdk: Fix machine name

ARM uses dashes instead of underscores for machine names. Fix imx25_pdk
which has not seen a release yet (so there is no legacy yet).

Cc: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1444445785-3648-1-git-send-email-crosthwaite.peter@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Added change to tests/ds1338-test.c to use new machine name]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>