dt-bindings: mfd: Document STM32 low power timer bindings

Add a subnode to STM low power timer bindings to support timer driver

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/st,stm32-lptimer.yaml

@@ -33,6 +33,9 @@ properties:
     items:
       - const: mux
 
+  interrupts:
+    maxItems: 1
+
   "#address-cells":
     const: 1
 
@@ -106,11 +109,13 @@ additionalProperties: false
 examples:
   - |
     #include <dt-bindings/clock/stm32mp1-clks.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
     timer@40002400 {
       compatible = "st,stm32-lptimer";
       reg = <0x40002400 0x400>;
       clocks = <&timer_clk>;
       clock-names = "mux";
+      interrupts-extended = <&exti 47 IRQ_TYPE_LEVEL_HIGH>;
       #address-cells = <1>;
       #size-cells = <0>;