PM: domains: Prevent power off for parent unless child is in deepest state

PM: domains: Prevent power off for parent unless child is in deepest state

A PM domain managed by genpd may support multiple idlestates (power-off
states). During genpd_power_off() a genpd governor may be asked to select
one of the idlestates based upon the dev PM QoS constraints, for example.

However, there is a problem with the behaviour around this in genpd. More
precisely, a parent-domain is allowed to be powered off, no matter of what
idlestate that has been selected for the child-domain.

For the stm32mp1 platform from STMicro, this behaviour doesn't play well.
Instead, the parent-domain must not be powered off, unless the deepest
idlestate has been selected for the child-domain. As the current behaviour
in genpd is quite questionable anyway, let's simply change it into what is
needed by the stm32mp1 platform.

If it surprisingly turns out that other platforms may need a different
behaviour from genpd, then we will have to revisit this to find a way to
make it configurable.

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