PM / Domains: Rename functions in genpd for power on/off

Currently the mix of genpd_poweron(), genpd_power_on(),
genpd_sync_poweron() and the ->power_on() callback, makes
a bit difficult to follow the path of execution. The similar
applies to the functions dealing with power off.

In a way to improve this understanding, let's do the following renaming:

genpd_power_on() ->  _genpd_power_on()
genpd_poweron() -> genpd_power_on()
genpd_sync_poweron() -> genpd_sync_power_on()

genpd_power_off() -> _genpd_power_off()
genpd_poweroff() -> genpd_power_off()
genpd_sync_poweroff() -> genpd_sync_power_off()
genpd_poweroff_unused() -> genpd_power_off_unused()

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