arm: socfpga: Change watchdog timeout

The current current watchdog timeout of 12 seconds is a bit small for
booting into Linux, especially when using a NFS based rootfs. So lets
change this timeout to a more defensive value of 30 seconds.

Also we now call the hw_watchdog_init() function so that we override
the value already configured from the Preloader.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Vince Bridgers <vbridger@opensource.altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Pavel Machek <pavel@denx.de>