watchdog: bcm47xx_wdt.c: add hard timer

The more recent devices have a watchdog timer which could be configured
for over 2 hours and not just 2 seconds like the first generation
devices. For those devices do not use the extra software timer, but
directly program the time into the register. This will automatically be
used if the timer supports more than a minute.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>