PM / Domains: Allow OF lookup for multi PM domain case from ->attach_dev()

A genpd provider that uses the ->attach_dev() callback to look up
resources for a device fails to do so when the device has multiple
PM domains attached.  That is because when genpd invokes the
->attach_dev() callback, it passes the allocated virtual device as
the in-parameter.

To address this problem, simply assign the dev->of_node for the
virtual device, based upon the original device's OF node.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>