gpio: rcar: Add DT binding for r8a77990

Add compatible string for R-Car E3 (r8a77990) in gpio-rcar.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>