xtensa: support s6000 gpio irqs and alternate function selection

Implement an irq chip to handle interrupts via gpio.  The GPIO chip
initialization function now takes a bitmask denoting pins that should
be configured for their alternate function.

changes compared to v1:
- fixed bug on edge interrupt configuration
- accommodated to function name change
- moved definition of VARIANT_NR_IRQS to this patch
- renamed __XTENSA_S6000_IRQ_H to _XTENSA_S6000_IRQ_H as requested

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/include/asm/gpio.h

@@ -38,14 +38,14 @@ static inline int gpio_cansleep(unsigned int gpio)
 	return __gpio_cansleep(gpio);
 }
 
+static inline int gpio_to_irq(unsigned int gpio)
+{
+	return __gpio_to_irq(gpio);
+}
+
 /*
  * Not implemented, yet.
  */
-static inline int gpio_to_irq(unsigned int gpio)
-{
-	return -ENOSYS;
-}
-
 static inline int irq_to_gpio(unsigned int irq)
 {
 	return -EINVAL;