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>

Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt

@@ -15,6 +15,7 @@ Required Properties:
     - "renesas,pfc-r8a7778": for R8A7778 (R-Mobile M1) compatible pin-controller.
     - "renesas,pfc-r8a7779": for R8A7779 (R-Car H1) compatible pin-controller.
     - "renesas,pfc-r8a7790": for R8A7790 (R-Car H2) compatible pin-controller.
+    - "renesas,pfc-r8a7791": for R8A7791 (R-Car M2) compatible pin-controller.
     - "renesas,pfc-sh7372": for SH7372 (SH-Mobile AP4) compatible pin-controller.
     - "renesas,pfc-sh73a0": for SH73A0 (SH-Mobile AG5) compatible pin-controller.