Add initial PFC support for the r8a73a4 SoC.

At this point only GPIO interface is supported, move to newer interfaces
planned as incremental changes.

Original authors are Morimoto-san with help from Yoshii-san, thanks to
them for the heavy lifting. Adjusted by Magnus to work together with
updated code in drivers/pinctrl.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Takashi Yoshii <takashi.yoshii.zj@renesas.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>