x86/gpio: Implement x86 gpio_to_irq convert function

We need this for x86 MID platforms where GPIO interrupts are
used. No special magic is needed so the default 1:1 behaviour
will do nicely.

Signed-off-by: Alek Du <[e-mail]>
Signed-off-by: Jacob Pan <[e-mail]>
Signed-off-by: Alan Cox <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/gpio.h

···
 	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)