PM / Runtime: GENERIC_SUBSYS_PM_OPS is gone

Update the documentation for the removal of GENERIC_SUBSYS_PM_OPS in commit
90363ddf

 ("PM: Drop generic_subsys_pm_ops")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>