PM / Domains: Fix compatible for domain idle state

Re-using idle state definition provided by arm,idle-state for domain
idle states creates a lot of confusion and limits further evolution of
the domain idle definition. To keep things clear and simple, define a
idle states for domain using a new compatible "domain-idle-state".

Fix existing PM domains code to look for the newly defined compatible.

Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>