Add a dedicated ARCH_STM32 for STM32 SoCs config. First STM32 Armv8 SoC
family is the STM32MP25 which is composed of STM32MP251, STM32MP253,
STM32MP255, STM32MP257 SoCs.
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>