PM / Domains: Rename is_async to one_dev_on for genpd_power_off()

The parameter name is_async, for genpd_power_off() gives a poor description
of its purpose. To clarify, let's rename it to one_dev_on and update the
documentation of it in the function header.

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