arm64: dts: qcom: msm8916: Add entry-method property for the idle-states node

The idle-states binding documentation[1] mentions that the
'entry-method' property is required on 64-bit platforms and must be set
to "psci".

[1] Documentation/devicetree/bindings/arm/idle-states.txt (see
idle-states node)
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <agross@kernel.org>

arch/arm64/boot/dts/qcom/msm8916.dtsi

@@ -158,6 +158,8 @@ L2_0: l2-cache {
 		};
 
 		idle-states {
+			entry-method = "psci";
+
 			CPU_SPC: spc {
 				compatible = "arm,idle-state";
 				arm,psci-suspend-param = <0x40000002>;