gpio: rcar: Add r8a7791 support to the DT bindings

The r8a7791 (R-Car M2) has a GPIO controller with additional features
compared to the generic renesas,gpio-rcar compatible devices. Add a
model-specific string to let the driver enable these features.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -2,10 +2,11 @@
 
 Required Properties:
 
-  - compatible: should be one of the following.
+  - compatible: should contain one of the following.
     - "renesas,gpio-r8a7778": for R8A7778 (R-Mobile M1) compatible GPIO controller.
     - "renesas,gpio-r8a7779": for R8A7779 (R-Car H1) compatible GPIO controller.
     - "renesas,gpio-r8a7790": for R8A7790 (R-Car H2) compatible GPIO controller.
+    - "renesas,gpio-r8a7791": for R8A7791 (R-Car M2) compatible GPIO controller.
     - "renesas,gpio-rcar": for generic R-Car GPIO controller.
 
   - reg: Base address and length of each memory resource used by the GPIO