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 <[e-mail]>
Signed-off-by: Fabrice Gasnier <[e-mail]>
Signed-off-by: Antonio Borneo <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/interrupt-controller/st,stm32-exti.yaml

···
         reg = <0x5000d000 0x400>;
     };
 
+  - |
     //Example 2
-    exti2: interrupt-controller@40013c00 {
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    exti2: interrupt-controller@5000d000 {
+        compatible = "st,stm32mp1-exti", "syscon";
+        interrupt-controller;
+        #interrupt-cells = <2>;
+        reg = <0x5000d000 0x400>;
+        interrupts-extended =
+            <&intc GIC_SPI 6 IRQ_TYPE_LEVEL_HIGH>,
+            <0>,
+            <&intc GIC_SPI 9 IRQ_TYPE_LEVEL_HIGH>;
+    };
+
+  - |
+    //Example 3
+    exti3: interrupt-controller@40013c00 {
         compatible = "st,stm32-exti";
         interrupt-controller;
         #interrupt-cells = <2>;