Update HAL.h

Reference: http://www.atmel.com/images/doc11057.pdf , page 272
To enable wathdog in Due in WT_MR register we need both
WDD: Watchdog Delta Value &  WDV: Watchdog Counter Value
WDV is WATCHDOG_INTERVAL. To simplify the setup WDD value should same as WATCHDOG_INTERVAL
But in startwatchdog() WDD is not included setup. This will cause the Due board to reset it self due to error in watchdog reset.
Adding WATCHDOG_INTERVAL<<16 in startWatchdog() will fix this issue.