dt-bindings: mfd: qcom-pm8xxx: Document qcom,pm8921 as fallback of qcom,pm8018

The PM8018 is used as compatible with PM8921 on the MDM9615, document this situation,
and an example section to validate this change.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v4-5-dac2dfaac703@linaro.org

Documentation/devicetree/bindings/mfd/qcom-pm8xxx.yaml

@@ -15,11 +15,15 @@ description: |
 
 properties:
   compatible:
-    enum:
-      - qcom,pm8018
-      - qcom,pm8058
-      - qcom,pm8821
-      - qcom,pm8921
+    oneOf:
+      - enum:
+          - qcom,pm8058
+          - qcom,pm8821
+          - qcom,pm8921
+      - items:
+          - enum:
+              - qcom,pm8018
+          - const: qcom,pm8921
 
   reg:
     maxItems: 1
@@ -52,4 +56,23 @@ required:
   - interrupt-controller
 
 additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+    ssbi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      pmic@0 {
+        compatible = "qcom,pm8921";
+        reg = <0>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+        interrupt-controller;
+        #interrupt-cells = <2>;
+
+        interrupt-parent = <&tlmm>;
+        interrupts = <32 IRQ_TYPE_EDGE_RISING>;
+      };
+    };
 ...