gpio: it87: add support for IT8772F 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: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>