drivers/exti: stm32: Use CMSIS IRQ defines instead of zephyr

stm32cube SDK provides defines for IRQ line numbers.
It was not possible to use them, since enum where not supported
by IRQ_CONNECT macro.
Use them in order to get rid of zephyr manually coded IRQ lines.
They will later be replaced by device tree definitions when
made available

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