hw/ptimer: Introduce ptimer_get_limit

Currently ptimer users are used to store copy of the limit value, because
ptimer doesn't provide facility to retrieve the limit. Let's provide it.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 8f1fa9f90d8dbf8086fb02f3b4835eaeb4089cf6.1464367869.git.digetx@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
