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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio-74x164.txt

···
       1 = active low
 - registers-number: Number of daisy-chained shift registers
 
+Optional properties:
+- enable-gpios: GPIO connected to the OE (Output Enable) pin.
+
 Example:
 
 gpio5: gpio5@0 {

drivers/gpio/gpio-74x164.c

···
  *  published by the Free Software Foundation.
  */
 
+#include <linux/gpio/consumer.h>
 #include <linux/init.h>
 #include <linux/mutex.h>
 #include <linux/spi/spi.h>
···
 	 * numbering, store the bytes in reverse order.
 	 */
 	u8			buffer[0];
+	struct gpio_desc	*gpiod_oe;
 };
 
 static int __gen_74x164_write_config(struct gen_74x164_chip *chip)
···
 	if (!chip)
 		return -ENOMEM;
 
+	chip->gpiod_oe = devm_gpiod_get_optional(&spi->dev, "enable",
+						 GPIOD_OUT_LOW);
+	if (IS_ERR(chip->gpiod_oe))
+		return PTR_ERR(chip->gpiod_oe);
+
+	gpiod_set_value_cansleep(chip->gpiod_oe, 1);
+
 	spi_set_drvdata(spi, chip);
 
 	chip->gpio_chip.label = spi->modalias;
···
 {
 	struct gen_74x164_chip *chip = spi_get_drvdata(spi);
 
+	gpiod_set_value_cansleep(chip->gpiod_oe, 0);
 	gpiochip_remove(&chip->gpio_chip);
 	mutex_destroy(&chip->lock);