PM: domains: Move the next_wakeup variable into the struct gpd_timing_data

If the corresponding genpd for the device doesn't use a governor, the
variable next_wakeup within the struct generic_pm_domain_data becomes
superfluous.

To avoid wasting memory, let's move it into the struct gpd_timing_data,
which is already being allocated based upon if there is governor assigned.

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