rtc: stm32: Fix copyright

Uniformize STMicroelectronics copyrights header

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
CC: Amelie Delaunay <amelie.delaunay@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>