dt-bindings/interrupt-controllers: Add compatible string for stm32h7

This patch updates stm32-exti documentation with stm32h7-exti
compatible string.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLudovic Barre <ludovic.barre@st.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

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

@@ -2,7 +2,9 @@ STM32 External Interrupt Controller
 
 Required properties:
 
-- compatible: Should be "st,stm32-exti"
+- compatible: Should be:
+    "st,stm32-exti"
+    "st,stm32h7-exti"
 - reg: Specifies base physical address and size of the registers
 - interrupt-controller: Indentifies the node as an interrupt controller
 - #interrupt-cells: Specifies the number of cells to encode an interrupt