arm64: Kconfig: Select STM32MP_EXTI on STM32 platforms

Use the new config flag to build the correct driver that will be
extracted from the old code.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240620083115.204362-4-antonio.borneo@foss.st.com