watchdog: omap: use watchdog_init_timeout instead of open coding it

Instead of (partly) open coding watchdog_init_timeout to determine the
inital timeout use the core function that exists for exactly this
purpose.

As a side effect the "timeout-sec" device-tree property is recognized now
(though currently unused in the omap device trees).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>