gpiolib: Add stubs for GPIO lookup functions

The gpio_device_find_by_() functions do not have stubs which means that if
they are referenced from code with an optiona dependency on gpiolib then
the code will fail to link. Add stubs for lookups via fwnode and label. I
have not added a stub for plain gpio_device_find() since it seems harder to
see a use case for that which does not depend on gpiolib.

With the addition of the GPIO reset controller (which lacks a gpiolib
dependency) to the arm64 defconfig this is causing build breaks for arm64
virtconfig in -next:

aarch64-linux-gnu-ld: drivers/reset/core.o: in function `__reset_add_reset_gpio_lookup':
 /build/stage/linux/drivers/reset/core.c:861:(.text+0xccc): undefined reference to `gpio_device_find_by_fwnode'
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

include/linux/gpio/driver.h

@@ -646,8 +646,6 @@ int devm_gpiochip_add_data_with_key(struct device *dev, struct gpio_chip *gc,
 struct gpio_device *gpio_device_find(const void *data,
 				int (*match)(struct gpio_chip *gc,
 					     const void *data));
-struct gpio_device *gpio_device_find_by_label(const char *label);
-struct gpio_device *gpio_device_find_by_fwnode(const struct fwnode_handle *fwnode);
 
 struct gpio_device *gpio_device_get(struct gpio_device *gdev);
 void gpio_device_put(struct gpio_device *gdev);
@@ -814,6 +812,9 @@ struct gpio_device *gpiod_to_gpio_device(struct gpio_desc *desc);
 int gpio_device_get_base(struct gpio_device *gdev);
 const char *gpio_device_get_label(struct gpio_device *gdev);
 
+struct gpio_device *gpio_device_find_by_label(const char *label);
+struct gpio_device *gpio_device_find_by_fwnode(const struct fwnode_handle *fwnode);
+
 #else /* CONFIG_GPIOLIB */
 
 #include <asm/bug.h>
@@ -843,6 +844,18 @@ static inline const char *gpio_device_get_label(struct gpio_device *gdev)
 	return NULL;
 }
 
+static inline struct gpio_device *gpio_device_find_by_label(const char *label)
+{
+	WARN_ON(1);
+	return NULL;
+}
+
+static inline struct gpio_device *gpio_device_find_by_fwnode(const struct fwnode_handle *fwnode)
+{
+	WARN_ON(1);
+	return NULL;
+}
+
 static inline int gpiochip_lock_as_irq(struct gpio_chip *gc,
 				       unsigned int offset)
 {