powerpc/gpio: support gpio_to_irq()

gpiolib returns -ENXIO if struct gpio_chip::to_irq isn't set, so it's
safe to always call.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/include/asm/gpio.h

@@ -38,12 +38,9 @@ static inline int gpio_cansleep(unsigned int gpio)
 	return __gpio_cansleep(gpio);
 }
 
-/*
- * Not implemented, yet.
- */
 static inline int gpio_to_irq(unsigned int gpio)
 {
-	return -ENOSYS;
+	return __gpio_to_irq(gpio);
 }
 
 static inline int irq_to_gpio(unsigned int irq)