drivers/interrupt_controller: stm32: Use LL API

exti driver implementation does not fit all SoCs because
some EXTI ip does not match stm32_exti register map provided.
Instead of providing exti register map for all SoCs, use LL API
which abstracts IP variations and enable uniform use of the drivers
on all STM32SoCs.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>