gpio: rcar: Add minimal runtime PM support

gpio: rcar: Add minimal runtime PM support

This is just enough to automatically enable the functional clock, if
present. Clock management during suspend/resume is still to be added.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: linux-gpio@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
(cherry picked from commit df0c6c80232f2ad442dcf79a3a420eb8ac624e42)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>