acpi/gsi: Add acpi_set_irq_model to initialize the GSI layer

In order to start embrassing irqdomains at the GSI level, introduce
a new initializer:

void acpi_set_irq_model(enum acpi_irq_model_id model,
			struct fwnode_handle *fwnode);

where:
- model is the value assigned to acpi_irq_model
- fwnode is the identifier for the irqdomain mapping
  GSI interrupts

As nobody calls this code yet, the current code is (mostly)
left in place.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-and-tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Tested-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: Graeme Gregory <graeme@xora.org.uk>
Cc: Jake Oshins <jakeo@microsoft.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Link: http://lkml.kernel.org/r/1444737105-31573-11-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>