sh-pfc: Initial r8a7790 PFC support

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>