gpio: rcar: Add r8a7793 and r8a7794 support

The device tree probing for R-Car M2-N (r8a7793) and R-Car E2 (r8a7794)
is added.

Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>