gpio: add bindings for Technologic I2C-FPGA gpio controller

Device tree binding documentation for Technologic's I2C-FPGA GPIO
controller.

Signed-off-by: Lucile Quirion <lucile.quirion@savoirfairelinux.com>
[Quirked in a reference to gpio.txt]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>