PM / Domains: Remove ->save|restore_state() callbacks

PM / Domains: Remove ->save|restore_state() callbacks

As a part of the ongoing consolidation of genpd, it's become questionable
whether clients actually needs to be able to assign their own set of
->save|restore_state() callbacks. Currently all users copes fine with the
default callbacks, so let's remove the configuration option and stick to
the default ones.

This enables further clarifications of the related code and let's also
rename pm_genpd_default_save|restore_state() into
__genpd_runtime_suspend|resume() to apply the rule of static functionnames
in genpd.

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