watchdog: stm32: Fix copyright

Uniformize STMicroelectronics copyrights header
Add SPDX identifier

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
CC: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Yannick Fertre <yannick.fertre@st.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>