sh-pfc: Add r8a73a4 pinmux support

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>