PM / Domains: dt: Add a power-domain-names property

Let's add a power-domain-names property, to allow consumer drivers to match
the power-domains specifiers via a list of power domain names. This follows
the same concept as for other similar DT bindings.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>