gpio: add parameter to allow the use named gpios

The gpio binding document says that new code should always use named
gpios. Patch 40b73183 added support to parse a list of gpios from child
nodes, but does not make it possible to use named gpios. This patch adds
the con_id property and implements it is done in gpiolib.c, where the
old-style of using unnamed gpios still works.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/consumer.h

@@ -110,6 +110,7 @@ struct fwnode_handle;
 struct gpio_desc *fwnode_get_named_gpiod(struct fwnode_handle *fwnode,
 					 const char *propname);
 struct gpio_desc *devm_get_gpiod_from_child(struct device *dev,
+					    const char *con_id,
 					    struct fwnode_handle *child);
 #else /* CONFIG_GPIOLIB */