PM / Domains: Add time accounting to various genpd states

This patch adds support to calculate the time spent by the generic
power domains in on and various idle states.

Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>