gpio-it87: add support for IT8786E Super I/O

From the datasheet, the GPIO interface is identical to IT8728 (same
description), so just add it to the same case as the other chip.

Signed-off-by: Vincent Prince <vincent.prince.fr@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>