dt-bindings: watchdog: Restrict timeout-sec to one number

Linux kernel expects only one number for the watchdog timeout and the
type is an array (defined in property-units.yaml in DT schema), so
restrict the property.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>