watchdog: stpmic1: Add STPMIC1 watchdog driver

The STPMIC1 PMIC embeds a watchdog which is disabled by default. As soon
as the watchdog is started, it must be refreshed periodically otherwise
the PMIC goes off.

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