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