PM / Domains: Prepare for adding support to remove PM domains

In order to remove PM domains safely from the list of PM domains,
it is necessary to adding locking for the PM domain list around any
places where devices or subdomains are added to a PM domain.

There are places where a reference to a PM domain is obtained via
calling of_genpd_get_from_provider() before adding the device or the
subdomain. In these cases a lock for the PM domain list needs to be
held around the call to of_genpd_get_from_provider() and the call to
add the device/subdomain. To avoid deadlocks by multiple attempts to
obtain the PM domain list lock, add functions genpd_add_device() and
genpd_add_subdomain() which require the user to hold the PM domain
list lock when calling.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>