dt-bindings: gpio: rcar: Document r8a77961 support

Document support for the GPIO controller in the Renesas R-Car M3-W+
(R8A77961) SoC.

Update all references to R-Car M3-W from "r8a7796" to "r8a77960", to
avoid confusion between R-Car M3-W (R8A77960) and M3-W+.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191205133912.6048-1-geert+renesas@glider.be


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>