stm32 EXTI driver - replace table of callbacks by one callback 

The only user of this API is the GPIO driver, and we know it's well behaved, so the NULL check isn't necessary IMO.

(as a matter of fact, we could replace this array-of-callbacks with a single one... but that's a different topic)

_Originally posted by @mathieuchopstm in https://github.com/zephyrproject-rtos/zephyr/pull/85508#discussion_r2144400157_

First the concept shall be created.