dt-bindings: watchdog: Document STM32 IWDG bindings

This adds documentation of device tree bindings for the STM32 IWDG
(Independent WatchDoG).

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>