PM / Domains: Introduce option to attach a device by name to genpd

For the multiple PM domain case, let's introduce a new function called
genpd_dev_pm_attach_by_name(). This allows a device to be associated with
its PM domain through genpd, by using a name based lookup.

Note that, genpd_dev_pm_attach_by_name() shall only be called by the driver
core / PM core, similar to how the existing dev_pm_domain_attach_by_id()
makes use of genpd_dev_pm_attach_by_id(). However, this is implemented by
following changes on top.

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>