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

arch/arm64/boot/dts/qcom/msm8939-pm8916.dtsi

 // SPDX-License-Identifier: GPL-2.0-only
+/*
+ * msm8939-pm8916.dtsi describes common properties (e.g. regulator connections)
+ * that apply to most devices that make use of the MSM8939 SoC and PM8916 PMIC.
+ * Many regulators have a fixed purpose in the original reference design and
+ * were rarely re-used for different purposes. Devices that deviate from the
+ * typical reference design should not make use of this include and instead add
+ * the necessary properties in the board-specific device tree.
+ */
 
 #include "msm8939.dtsi"
 #include "pm8916.dtsi"