pinctrl: sh-pfc: r8a7796: Add DU support

Only the DU parallel RGB output signals are included, HDMI and TCON pins
will be added in separate groups. Based on a similar patch from Laurent
Pinchart for the r8a7795 PFC driver.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>