soc: mediatek: Use GENPD_FLAG_ACTIVE_WAKEUP

Set the newly introduced GENPD_FLAG_ACTIVE_WAKEUP, which allows to
remove the driver's own flag-based callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>