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.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>