PM / Domains: Rename pm_genpd_sync_poweron|poweroff()

These are internal static functions to genpd. Let's conform to the naming
rules, by dropping the "pm_" prefix from these.

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