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: NDaniel Glöckner <dg@emlix.com>
Signed-off-by: NJohannes Weiner <jw@emlix.com>
Signed-off-by: NChris Zankel <chris@zankel.net>