dt-bindings: mfd: qcom: pm8xxx: Add pm8018 compatible

Add missing compatible for the PM8018 model.

Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211020231646.278706-1-david@ixit.cz