PM: domains: Enable dev_pm_genpd_suspend|resume() for suspend-to-idle

The dev_pm_genpd_suspend|resume() have so far only been used during the
syscore suspend/resume phases. However, during suspend-to-idle, where the
syscore phases doesn't exist, similar operations are sometimes needed.

An existing example are the timekeeping_suspend|resume() functions, which
are being called both through a registered syscore ops during the syscore
phases, but also as regular functions calls from cpuidle (via
tick_freeze()) during suspend-to-idle.

For similar reasons, let's enable the dev_pm_genpd_suspend|resume() APIs to
be re-used for corresponding CPU devices that are attached to a genpd,
during suspend-to-idle.

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