watchdog: mtk: allow setting timeout in devicetree

watchdog_init_timeout() will allways pick timeout_param since it
defaults to a valid timeout.

By following best practice described in
Documentation/watchdog/watchdog-kernel-api.txt, it also
let us to set timout-sec property in devicetree.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>