PM / Domains: Allow drivers to attach PM domain callbacks to devices at any time

Hi,

This is a follow-up of the discussion I had with Arnd and Magnus during
the LCJ last month.

Namely, one of the limitations of the current generic PM domains code is that
pm_genpd_add_callbacks() has to be executed after the device in question has
been added to a PM domain.  This means that, if a driver wants to run this
routine from its .probe() callback, it cannot be registered before adding
the device to the PM domain, which potentially may be a problem when device
trees get involved into the system initialization.

For this reason, it is desirable to allow pm_genpd_add_callbacks() to be called
for devices that haven't been added to any generic PM domains yet and the
following patches attempt to make that happen.

[1/2] Add reference counter for objects used to store PM domains data related
      to devices.
[2/2] Make it possible to call pm_genpd_add_callbacks() before the device is
      added to a PM domain.

Thanks,
Rafael

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