gpio: 74x164: Introduce 'enable-gpios' property

74HC595 has an /OE (output enable) pin that can be controlled by a GPIO.

Introduce an optional property called 'enable-gpios' that allows
controlling the /OE pin.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>