When adding a device to a genpd, a struct generic_pm_domain_data is
allocated per device.

Verify that there are no existing generic_pm_domain_data for the device
we are about to add, since that tells us it has already been added to a
genpd.

When genpd supported PM domain device callbacks, this was a valid
scenario. Now it isn't so let's return an error code.
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>