sh-pfc: Add renesas,pfc-r8a7791 to binding documentation

The driver already supports the r8a7791 SoC, and "renesas,pfc-r8a7791"
is already in use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>