dt-bindings: rtc: qcom-pm8xxx: document qcom,pm8921-rtc as fallback of qcom,pm8018-rtc

The PM8018 RTC is used as compatible with PM8921 RTC on the MDM9615,
document this situation.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v5-1-bbb120c6766a@linaro.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -11,12 +11,16 @@ maintainers:
 
 properties:
   compatible:
-    enum:
-      - qcom,pm8058-rtc
-      - qcom,pm8921-rtc
-      - qcom,pm8941-rtc
-      - qcom,pm8018-rtc
-      - qcom,pmk8350-rtc
+    oneOf:
+      - enum:
+          - qcom,pm8058-rtc
+          - qcom,pm8921-rtc
+          - qcom,pm8941-rtc
+          - qcom,pmk8350-rtc
+      - items:
+          - enum:
+              - qcom,pm8018-rtc
+          - const: qcom,pm8921-rtc
 
   reg:
     minItems: 1