ARM: stm32: Fix copyright

Uniformize STMicroelectronics copyrights header
Add SPDX identifier

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
CC: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>