dt-bindings: power: Convert domain-idle-states bindings to json-schema

While converting to the json-schema, let's also take the opportunity to
further specify/clarify some more details about the DT binding.

For example, let's define the label where to put the states nodes, set a
pattern for nodename of the state nodes and finally add an example.

Fixes: a3f048b5 ("dt: psci: Update DT bindings to support hierarchical PSCI states")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[robh: drop type refs from standard unit properties]
Signed-off-by: Rob Herring <robh@kernel.org>