ptimer: Add TRIGGER_ONLY_ON_DECREMENT policy option

The CMSDK timer behaviour is that an interrupt is triggered when the
counter counts down from 1 to 0; however one is not triggered if the
counter is manually set to 0 by a guest write to the counter register.
Currently ptimer can't handle this; add a policy option to allow
a ptimer user to request this behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20180703171044.9503-2-peter.maydell@linaro.org