MAINTAINERS: Update stm32 entry

Add mailing list for stm32 architecture.
Add "stm" pattern to not miss some drivers/directories when asking for maintainers.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Olof Johansson <olof@lixom.net>