watchdog: davinci: add "timeout-sec" property

Since Davinci WDT has been switched to use WDT core, it became able
to support timeout-sec property, so add it to it's binding description.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>