PM: domains: Add GENPD_FLAG_RPM_ALWAYS_ON for the always-on governor

Rather than relying on the genpd provider to set the corresponding flag,
GENPD_FLAG_RPM_ALWAYS_ON, when the always-on governor is being used, let's
add it in pm_genpd_init(). In this way, it starts to benefits all genpd
providers immediately.

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