arm64: dts: qcom: msm8916: Conform to the domain-idle-state binding

As the domain-idle-state DT binding got converted to the json-schema, a few
minor changes were introduced. For example, the states needs to be
specified within a separate node, named domain-idle-states. Let's conform
to the updated binding.

This also silence the below errors for msm8916 from of 'make dtbs_check':
idle-states: cluster-retention:compatible:0: 'arm,idle-state' was expected
idle-states: cluster-gdhs:compatible:0: 'arm,idle-state' was expected
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200326103932.5809-2-ulf.hansson@linaro.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

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

@@ -165,6 +165,9 @@ CPU_SLEEP_0: cpu-sleep-0 {
 				min-residency-us = <2000>;
 				local-timer-stop;
 			};
+		};
+
+		domain-idle-states {
 
 			CLUSTER_RET: cluster-retention {
 				compatible = "domain-idle-state";