dt-bindings: pinctrl: renesas,pfc: Document r8a779a0 PFC support

Document PFC support for the R-Car V3U (R8A779A0) SoC.

Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Link: https://lore.kernel.org/r/20210112165912.30876-7-uli+renesas@fpond.eu


[geert: increase reg maxItems to 10]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>