x86, irq: Introduce mechanisms to support dynamically allocate IRQ for IOAPIC

Currently x86 support identity mapping between GSI(IOAPIC pin) and IRQ
number, so continous IRQs at low end are statically allocated to IOAPICs
at boot time. This design causes trouble to support IOAPIC hotplug.

This patch implements basic mechanism to dynamically allocate IRQ on
demand for IOAPIC pins by using irqdomain framework.

It first adds several fields into struct ioapic to support irqdomain.
Then it implements an algorithm to dynamically allocate IRQ number
for IOAPIC pins on demand.

Currently it supports three types of irqdomain:
1) LEGACY: used to support IOAPIC hosting legacy IRQs and building
   identity mapping for legacy IRQs. A speical case, we dynamically
   allocate IRQ number for IOAPIC pin which has GSI number below
   nr_legacy_irqs() but isn't legacy IRQ. This is for backward
   compatibility and avoid regression.
2) STRICT: build identity mapping between GSI and IRQ nubmer.
3) DYNAMIC: dynamically allocate IRQ number for IOAPIC pin on demand.

Legacy(ISA) IRQs is not managed by irqdomain because there may be
multiple pins sharing the same IRQ number and current irqdomain only
supports 1:1 mapping between pins and IRQ.

Signed-off-by: Jiang Liu <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Grant Likely <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Pavel Machek <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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