PM: domains: Rename pm_genpd_syscore_poweroff|poweron()

To better describe what the pm_genpd_syscore_poweroff|poweron() functions
actually do, let's rename them to dev_pm_genpd_suspend|resume() and update
the rather few callers of them accordingly (a couple of clocksource
drivers).

Moreover, let's take the opportunity to add some documentation of these
exported functions, as that is currently missing.

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