ARM: shmobile: Convert to genpd flags for PM clocks for r8a7779

Instead of using the dev_ops ->stop|start() callbacks for genpd, let's
convert to use genpd's flag field and set it to GENPD_FLAG_PM_CLK.

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