PM / Domains: Fix DT example

The power-domain provider's #power-domain-cells field is set to 0 and
yet the children is using an index to point the power domain. Fix it by
removing the index field.

Fixes: 70bb510e (dt/bindings / PM/Domains: Update binding for PM domain idle states)
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>