i2c: stm32: Fix copyrights

Uniformize STMicroelectronics copyrights headers and add SPDX
identifier.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Acked-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>