Document the compatible value for the GPIO block in the Renesas R-Car
V3U (R8A779A0) SoC.

While this GPIO block is mostly compatible with GPIO blocks on R-Car
Gen3 SoCs, there are small differences, and one of the new registers
needs to be configured differently from its initial reset state.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>