genirq: Allow the affinity of a percpu interrupt to be set/retrieved

In order to prepare the genirq layer for the concept of partitionned
percpu interrupts, let's allow an affinity to be associated with
such an interrupt. We introduce:

- irq_set_percpu_devid_partition: flag an interrupt as a percpu-devid
  interrupt, and associate it with an affinity
- irq_get_percpu_devid_partition: allow the affinity of that interrupt
  to be retrieved.

This will allow a driver to discover which CPUs the per-cpu interrupt
can actually fire on.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Link: http://lkml.kernel.org/r/1460365075-7316-3-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irqdesc.h

@@ -66,6 +66,7 @@ struct irq_desc {
 	int			threads_handled_last;
 	raw_spinlock_t		lock;
 	struct cpumask		*percpu_enabled;
+	const struct cpumask	*percpu_affinity;
 #ifdef CONFIG_SMP
 	const struct cpumask	*affinity_hint;
 	struct irq_affinity_notify *affinity_notify;