PM / Domains: Implement the ->start() callback for genpd

To allow a subsystem/driver to explicitly start its device from genpd's
point view, let's implement the ->start() callback in the struct
dev_pm_domain that corresponds to the genpd.

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