MIPS: lantiq: make use of __gpio_to_irq

The gpio_chip struct allows us to set a .to_irq callback. Once this is set
we can rely on the generic __gpio_to_irq() function to map gpio->irq allowing
more than one gpio_chip to register an interrupt
Signed-off-by: NJohn Crispin <blogic@openwrt.org>

arch/mips/include/asm/mach-lantiq/gpio.h

 #ifndef __ASM_MIPS_MACH_LANTIQ_GPIO_H
 #define __ASM_MIPS_MACH_LANTIQ_GPIO_H
 
-static inline int gpio_to_irq(unsigned int gpio)
-{
-	return -1;
-}
+#define gpio_to_irq __gpio_to_irq
 
 #define gpio_get_value __gpio_get_value
 #define gpio_set_value __gpio_set_value