arm64: dts: qcom: msm8939-pm8916: Clarify purpose

Add the same comment to msm8939-pm8916.dtsi that was added for the
MSM8916 variant in commit f1932649 ("arm64: dts: qcom:
msm8916-pm8916: Clarify purpose").

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230530-msm8939-regulators-v1-6-a3c3ac833567@gerhold.net