PM: domains: Fix handling of unavailable/disabled idle states

Platforms can provide the information about the availability of each
idle states via status flag. Platforms may have to disable one or more
idle states for various reasons like broken firmware or other unmet
dependencies.

Fix handling of such unavailable/disabled idle states by ignoring them
while parsing the states.

Fixes: a3381e3a ("PM / domains: Fix up domain-idle-states OF parsing")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>