microblaze: support gpio_to_irq()

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

Based on PPC patch
"powerpc/gpio: support gpio_to_irq()"
78331aded913d76c7ff996a1a0747d54b057460b

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/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)