PM / Domains: Rename *pm_genpd_poweron|poweroff()

This is another step to get consistent names of functions in genpd. Let's
rename the internal *pm_genpd_poweron|poweroff() into
*genpd_poweron|poweroff().

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