PM / Domains: Rename __pm_genpd_alloc|free_dev_data()

In a step to get consistent names of functions in genpd, rename
the internal __pm_genpd_alloc|free_dev_data() into
gendp_alloc|free_dev_data().

As discussed on the linux-pm list, let's move towards the following
name rules:

Internal functions:
genpd_*
_genpd_*
__genpd_*

External functions:
pm_genpd_*
_pm_genpd_*
__pm_genpd_*

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>