gpio: Add DT bindings for Gateworks PLD GPIO

This adds device tree bindings for the Gateworks PLD
GPIO chip, a simple I2C GPIO controller.

Cc: devicetree@vger.kernel.org
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>