PM / Domains: dt: Allow power-domain property to be a list of specifiers

To be able to describe topologies where devices are partitioned across
multiple power domains, let's extend the power-domain property to allow
being a list of PM domain specifiers.

Suggested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>