watchdog: bcm47xx_wdt.c: rename ops methods

Rename the methods registered to struct watchdog_ops bcm47xx_wdt_ops in
order to add an other struct watchdog_ops using different ops in the
next patch.
Also rename WDT_MAX_TIME to WDT_SOFTTIMER_MAX.

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