x86/xen: Treat SCI interrupt as normal GSI interrupt

Currently Xen Domain0 has special treatment for ACPI SCI interrupt,
that is initialize irq for ACPI SCI at early stage in a special way as:
xen_init_IRQ()
	->pci_xen_initial_domain()
		->xen_setup_acpi_sci()
			Allocate and initialize irq for ACPI SCI

Function xen_setup_acpi_sci() calls acpi_gsi_to_irq() to get an irq
number for ACPI SCI. But unfortunately acpi_gsi_to_irq() depends on
IOAPIC irqdomains through following path
acpi_gsi_to_irq()
	->mp_map_gsi_to_irq()
		->mp_map_pin_to_irq()
			->check IOAPIC irqdomain

For PV domains, it uses Xen event based interrupt manangement and
doesn't make uses of native IOAPIC, so no irqdomains created for IOAPIC.
This causes Xen domain0 fail to install interrupt handler for ACPI SCI
and all ACPI events will be lost. Please refer to:
https://lkml.org/lkml/2014/12/19/178

So the fix is to get rid of special treatment for ACPI SCI, just treat
ACPI SCI as normal GSI interrupt as:
acpi_gsi_to_irq()
	->acpi_register_gsi()
		->acpi_register_gsi_xen()
			->xen_register_gsi()

With above change, there's no need for xen_setup_acpi_sci() anymore.
The above change also works with bare metal kernel too.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Tony Luck <tony.luck@intel.com>
Cc: xen-devel@lists.xenproject.org
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/1421720467-7709-2-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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