PM / Domains: Add generic data pointer to struct genpd_power_state

Add a data pointer to the genpd_power_state struct, to allow a genpd
backend driver to store per-state specific data. To introduce the
pointer, change the way genpd deals with freeing of the corresponding
allocated data.

More precisely, clarify the responsibility of whom that shall free the
data, by adding a ->free_states() callback to the generic_pm_domain
structure. The one allocating the data will be expected to set the
callback, to allow genpd to invoke it from genpd_remove().

Co-developed-by: Lina Iyer <lina.iyer@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>