watchdog: sp805: add 'timeout-sec' DT property support

Add support for optional devicetree property 'timeout-sec'.
'timeout-sec' is used in the driver if specified in devicetree.
Otherwise, fall back to driver default, i.e., 60 seconds

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>