dt-bindings: interrupt-controller: stm32-exti: Add irq mapping to parent

The mapping of EXTI events to its parent interrupt controller is both SoC
and instance dependent.

The current implementation requires adding a new mapping table to the
driver's code and a new compatible for each new EXTI instance.

To avoid that use the interrupts-extended property to list, for each EXTI
event, the associated parent interrupt.

Co-developed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240415134926.1254428-3-antonio.borneo@foss.st.com