pinctrl: sh-pfc: r8a7794: Add DU pin groups

r8a7794 PFC DU support from the R-Car Gen2 v1.9.4 BSP

[Magnus: added the description, added missing dot clock output signals,
separated CDE and DISP signals, broke out the ODDF signal from the sync
group.]

[Sergei: resolved rejects, folded in Magnus' patches, killed empty lines,
reordered pin/mux arrays and pin groups, fixed up some comments to the pin
arrays, removed the "du" function splitting its groups between the "du0"
and "du1" functions.]

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>