gpiolib: fix up function prototypes etc

Commit 69e1601bca88809dc118abd1becb02c15a02ec71
"gpiolib: provide provision to register pin ranges"

Got most of it's function prototypes wrong, so fix this up by:

- Moving the void declarations into static inlines in
  <linux/gpio.h> (previously the actual prototypes were declared
  here...)

- Declare the gpiochip_add_pin_range() and
  gpiochip_remove_pin_ranges() functions in <asm-generic/gpio.h>
  together with the pin range struct declaration itself.

- Actually only implement these very functions in gpiolib.c
  if CONFIG_PINCTRL is set.

- Additionally export the symbols since modules will need to
  be able to do this.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio.h

@@ -72,9 +72,9 @@ static inline int irq_to_gpio(unsigned int irq)
 	return -EINVAL;
 }
 
-#endif
+#endif /* ! CONFIG_ARCH_HAVE_CUSTOM_GPIO_H */
 
-#else
+#else /* ! CONFIG_GENERIC_GPIO */
 
 #include <linux/kernel.h>
 #include <linux/types.h>
@@ -231,9 +231,21 @@ static inline int irq_to_gpio(unsigned irq)
 	return -EINVAL;
 }
 
-void gpiochip_add_pin_range(struct gpio_chip *chip, const char *pinctl_name,
-		unsigned int pin_base, unsigned int npins);
-void gpiochip_remove_pin_ranges(struct gpio_chip *chip);
-#endif
+#ifdef CONFIG_PINCTRL
+
+static inline void
+gpiochip_add_pin_range(struct gpio_chip *chip, const char *pinctl_name,
+		       unsigned int pin_base, unsigned int npins)
+{
+}
+
+static inline void
+gpiochip_remove_pin_ranges(struct gpio_chip *chip)
+{
+}
+
+#endif /* CONFIG_PINCTRL */
+
+#endif /* ! CONFIG_GENERIC_GPIO */
 
 #endif /* __LINUX_GPIO_H */