arm64: Kconfig.platforms: Enable STM32_EXTI for ARCH_STM32

ARCH_STM32 needs to use STM32 EXTI interrupt controller for GPIO and wakeup
interrupts.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240415134926.1254428-8-antonio.borneo@foss.st.com