hw/ptimer: Add "wraparound after one period" policy

hw/ptimer: Add "wraparound after one period" policy

Currently, periodic counter wraps around immediately once counter reaches
"0", this is wrong behaviour for some of the timers, resulting in one period
being lost. Add new ptimer policy that provides correct behaviour for such
timers, so that counter stays with "0" for a one period before wrapping
around.

Signed-off-by: Dmitry Osipenko <[email protected]>
Message-id: f22a670cf1f4be298b31640cb5f4be1df0f20ab6.1475421224[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>