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>

MAINTAINERS

@@ -1724,6 +1724,7 @@ F:	drivers/ata/ahci_st.c
 
 ARM/STM32 ARCHITECTURE
 M:	Maxime Coquelin <mcoquelin.stm32@gmail.com>
+M:	Alexandre Torgue <alexandre.torgue@st.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mcoquelin/stm32.git