clock: Add ClockPreUpdate callback event type

Add a new callback event type ClockPreUpdate, which is called on
period changes before the period is updated.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219144617.4782-3-peter.maydell@linaro.org

include/hw/clock.h

@@ -30,6 +30,7 @@ OBJECT_DECLARE_SIMPLE_TYPE(Clock, CLOCK)
  */
 typedef enum ClockEvent {
     ClockUpdate = 1, /* Clock period has just updated */
+    ClockPreUpdate = 2, /* Clock period is about to update */
 } ClockEvent;
 
 typedef void ClockCallback(void *opaque, ClockEvent event);