PM / domains: Remove the pm_genpd_add|remove_callbacks APIs

There are no users of these APIs. To simplify the generic power domain
let's remove them.

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