I think an 555 is not good solution because it works by level not in rise or fall edge:

If the signal send by the 8052 dont rises and falls the 555 timer dont start count and stays in the non reset condition for 8052.

And when 555 makes the reset you must end the reset signal if not, the 8052 will stay in reset condition forever. This problem can solved with capacitor and resistor but the control of reset lengt is not good.

I think if you want a good WatchDog use an specific chip or internal features of many 52'.

Bye