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>