PM / Domains: Enable genpd_dev_pm_attach_by_id|name() for single PM domain

If a call to dev_pm_domain_attach() succeeds to attach a device to its
single PM domain, the important point is to prevent subsequent
dev_pm_domain_attach_by_name|id() calls from failing. That is done by
checking the dev->pm_domain pointer and then returning -EEXIST, rather
than continuing to call genpd_dev_pm_attach_by_id|name().

For this reason, enable genpd_dev_pm_attach_by_id|name() to be used for
single PM domains too. This simplifies future users, so they only need
to use dev_pm_domain_attach_by_id|name() instead of having to combine
it with dev_pm_domain_attach().

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>