PM: domains: log failures to register always-on domains

Always-on PM domains must be on during initialisation or the domain is
currently silently rejected.

Print an error message in case an always-on domain is not on to make it
easier to debug drivers getting this wrong (e.g. by setting an always-on
genpd flag without making sure that the state matches).

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>