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>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

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

@@ -39,6 +39,8 @@ properties:
   "#size-cells":
     const: 0
 
+  wakeup-source: true
+
   pwm:
     type: object
 
@@ -81,6 +83,16 @@ patternProperties:
     required:
       - compatible
 
+  timer:
+    type: object
+
+    properties:
+      compatible:
+        const: st,stm32-lptimer-timer
+
+    required:
+      - compatible
+
 required:
   - "#address-cells"
   - "#size-cells"
@@ -115,6 +127,10 @@ examples:
       counter {
         compatible = "st,stm32-lptimer-counter";
       };
+
+      timer {
+        compatible = "st,stm32-lptimer-timer";
+      };
     };
 
 ...