dt-bindings: watchdog: document STPMIC1 PMIC watchdog

The STPMIC1 PMIC embeds a watchdog which is disabled by default.
In case of watchdog, the PMIC goes off.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>