STM32 EXTIv1 driver.

Driver capability:

- Support for the EXTI peripheral.

The file registry must export:

STM32_EXTI_NUM_LINES        - Number of EXTI lines, it can be between 0 and 63.
STM32_EXTI_IMR1_MASK        - Mask of the fixed lines that must not be
                              handled by the driver (0..31).
STM32_EXTI_IMR2_MASK        - Mask of the fixed lines that must not be
                              handled by the driver (32..63). Only required
                              if STM32_EXTI_NUM_LINES is greater than 32.