MAINTAINERS: update STM32 maintainers list

I will have less time to work on STM32 platform, so I propose
Alexandre as co-maintainer.

Alex is working in the STMicroelectronics division in charge of STM32
family, so he will have access to all technical information and
hardware.

Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>