gpio-rcar: document R8A77970 bindings

Renesas R-Car V3M (R8A77970) SoC also has the R-Car gen3 compatible GPIO
controllers, so document the SoC specific bindings.
Signed-off-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/renesas,gpio-rcar.txt

@@ -14,6 +14,7 @@ Required Properties:
     - "renesas,gpio-r8a7794": for R8A7794 (R-Car E2) compatible GPIO controller.
     - "renesas,gpio-r8a7795": for R8A7795 (R-Car H3) compatible GPIO controller.
     - "renesas,gpio-r8a7796": for R8A7796 (R-Car M3-W) compatible GPIO controller.
+    - "renesas,gpio-r8a77970": for R8A77970 (R-Car V3M) compatible GPIO controller.
     - "renesas,rcar-gen1-gpio": for a generic R-Car Gen1 GPIO controller.
     - "renesas,rcar-gen2-gpio": for a generic R-Car Gen2 or RZ/G1 GPIO controller.
     - "renesas,rcar-gen3-gpio": for a generic R-Car Gen3 GPIO controller.