irqchip/stm32mp-exti: Allow building as module

Allow to build the driver as a module by adding the necessarily hooks in
Kconfig and in the driver's code.

Since all the probe dependencies linked to this driver have already been
fixed, remove the not longer relevant 'arch_initcall'.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240620083115.204362-7-antonio.borneo@foss.st.com