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

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.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>