STM32 EXTIv1 driver.

Driver capability:

- Support for the EXTI peripheral.

The file registry must export:

STM32_EXTI_HAS_GROUP2       - True if lines from 32 to 63 are present.
STM32_EXTI_SEPARATE_RF      - True if EXTI has separate status registers
                              for falling and raising edges.
STM32_EXTI_IMR1_MASK        - Mask of the fixed lines that must not be
                              configured by the driver (0..31).
STM32_EXTI_IMR2_MASK        - Mask of the fixed lines that must not be
                              configured by the driver (32..63). Only required
                              if STM32_EXTI_NUM_LINES is greater than 32.