PM / Domains: Make pm_genpd_{add,remove}_subdomain() available to modules

Export these symbols so they can be used in loadable kernel
modules.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Rajendra Nayak <rnayak@codeaurora.org>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>