PM: domains: Allocate gpd_timing_data dynamically based on governor

If a genpd doesn't have an associated governor assigned, there's really no
point to allocate the per device gpd_timing_data, as the data isn't being
used by a governor anyway.

To avoid wasting memory, let's therefore convert the corresponding td
variable in the struct generic_pm_domain_data into a pointer and manage the
allocation of its data dynamically.

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