irqchip: Add per-cpu interrupt partitioning library

We've unfortunately started seeing a situation where percpu interrupts
are partitioned in the system: one arbitrary set of CPUs has an
interrupt connected to a type of device, while another disjoint
set of CPUs has the same interrupt connected to another type of device.

This makes it impossible to have a device driver requesting this interrupt
using the current percpu-interrupt abstraction, as the same interrupt number
is now potentially claimed by at least two drivers, and we forbid interrupt
sharing on per-cpu interrupt.

A solution to this is to turn things upside down. Let's assume that our
system describes all the possible partitions for a given interrupt, and
give each of them a unique identifier. It is then possible to create
a namespace where the affinity identifier itself is a form of interrupt
number. At this point, it becomes easy to implement a set of partitions
as a cascaded irqchip, each affinity identifier being the HW irq.

This allows us to keep a number of nice properties:
- Each partition results in a separate percpu-interrupt (with a restrictied
  affinity), which keeps drivers happy.
- Because the underlying interrupt is still per-cpu, the overhead of
  the indirection can be kept pretty minimal.
- The core code can ignore most of that crap.

For that purpose, we implement a small library that deals with some of
the boilerplate code, relying on platform-specific drivers to provide
a description of the affinity sets and a set of callbacks.

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/1460365075-7316-4-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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