dt-bindings: gpio: Add renesas,em-gio bindings

Document Device Tree bindings for the Renesas EMMA Mobile General
Purpose I/O Interface.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200519081157.29095-1-geert+renesas@glider.be


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