watchdog: sama5d4: fix WDD value to be always set to max

WDD value must be always set to max (0xFFF) otherwise the hardware
block will reset the board on the first ping of the watchdog.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>