hw/ptimer: Introduce timer policy feature

hw/ptimer: Introduce timer policy feature

Some of the timer devices may behave differently from what ptimer
provides. Introduce ptimer policy feature that allows ptimer users to
change default and wrong timer behaviour, for example to continuously
trigger periodic timer when load value is equal to "0".

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Message-id: 994cd608ec392da6e58f0643800dda595edb9d97.1473252818.git.digetx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>