pwm: rcar: Use PM Runtime to control module clock

Runtime PM API (pm_runtime_get_sync/pm_runtime_put) should be used
to control module clock instead of clk_prepare_enable and
clk_disable_unprepare.

Signed-off-by: Hien Dang <hien.dang.eb@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>