MAINTAINERS: add entry for ARM/STM32 ARCHITECTURE

STM32 SoCs based on Armv8 have been added to the STM32 family. Those new
SoCs are maintained as legacy STM32 MPU.
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

MAINTAINERS

@@ -2849,6 +2849,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/atorgue/stm32.git stm32-nex
 F:	arch/arm/boot/dts/stm32*
 F:	arch/arm/mach-stm32/
 F:	drivers/clocksource/armv7m_systick.c
+F:	arch/arm64/boot/dts/st/
 N:	stm32
 N:	stm