drivers: interrupt_controller: change dt macro

Replace the use of `DT_NODELABEL(exti)` which depends on the node label
with the defined EXTI_NODE macro using the instance with the
`st_stm32_exti` compatible.
Since both macros point to the same node, this change doesn't affect
the code logic, but makes it independent of the node label, in
preparation for the addition of the STM32MP2 exti nodes.

Signed-off-by: Youssef Zini <youssef.zini@savoirfairelinux.com>