PM / Domains: Make pm_genpd_init() available to modules

Export symbol pm_genpd_init so it can be used in loadable
kernel modules

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>