SAMV7: Watchdog: fix Forbidden Window Value

According the Datasheet the WDD Value is the lower bound of a so called Forbidden Window and to disable this we have to set the WDD
Value greater than or equal to the WDV Value.  This seems to be a bug in the datasheet. It looks like we have to set it to a greater value than the WDV to realy disable this Thing.  When triggering the Watchdog faster than the (very slow) clock source of the Watchdog fires, this Forbidden Window Feature resets the System if WDD equals to WDV.

This Changeset disables the Forbidden Window by setting the WDD Value to the Maximum (0xfff) Value possible.