gpiolib: allow to specify the firmware node in struct gpio_chip

Software nodes allow us to represent hierarchies for device components
that don't have their struct device representation yet - for instance:
banks of GPIOs under a common GPIO expander. The core gpiolib core
however doesn't offer any way of passing this information from the
drivers.

This extends struct gpio_chip with a pointer to fwnode that can be set
by the driver and used to pass device properties for child nodes.

This is similar to how we handle device-tree sub-nodes with
CONFIG_OF_GPIO enabled.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -593,7 +593,7 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 			       struct lock_class_key *lock_key,
 			       struct lock_class_key *request_key)
 {
-	struct fwnode_handle *fwnode = gc->parent ? dev_fwnode(gc->parent) : NULL;
+	struct fwnode_handle *fwnode = NULL;
 	struct gpio_device *gdev;
 	unsigned long flags;
 	int base = gc->base;
@@ -601,6 +601,11 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 	int ret = 0;
 	u32 ngpios;
 
+	if (gc->fwnode)
+		fwnode = gc->fwnode;
+	else if (gc->parent)
+		fwnode = dev_fwnode(gc->parent);
+
 	/*
 	 * First: allocate and populate the internal stat container, and
 	 * set up the struct device.

include/linux/gpio/driver.h

@@ -289,6 +289,7 @@ struct gpio_irq_chip {
  *	number or the name of the SoC IP-block implementing it.
  * @gpiodev: the internal state holder, opaque struct
  * @parent: optional parent device providing the GPIOs
+ * @fwnode: optional fwnode providing this controller's properties
  * @owner: helps prevent removal of modules exporting active GPIOs
  * @request: optional hook for chip-specific activation, such as
  *	enabling module power and clock; may sleep
@@ -377,6 +378,7 @@ struct gpio_chip {
 	const char		*label;
 	struct gpio_device	*gpiodev;
 	struct device		*parent;
+	struct fwnode_handle	*fwnode;
 	struct module		*owner;
 
 	int			(*request)(struct gpio_chip *gc,