PM / Domains: Constify genpd pointer

Mark pointer to struct generic_pm_domain const (either passed in
argument or used localy in a function), whenever it is not modifed by
the function itself.

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