watchdog: st_wdt: Add new driver for ST's LPC Watchdog

Signed-off-by: David Paris <david.paris@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>