[WATCHDOG] orion5x_wdt: Add shutdown callback, use watchdog ping function

* Added a callback to disable the watchdog on shutdown.
* Use a separate ping function to reduce the number of register accesses
  if the watchdog is already enabled and just needs to be reloaded.
* Minor cleanup of function names.

Signed-off-by: Thomas Reitmayr <treitmayr@devbase.at>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>