Add initial PFC support for the r8a7790 SoC.

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

Original authors is Koji Matsuoka-san, thanks for him
and his team for the heavy lifting. Adjusted by Magnus
to work together with updated code in drivers/pinctrl.
Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@rms.renesas.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>