To support IOAPIC hotplug, we need to allocate PCI IRQ resources on demand
and free them when not used anymore.

Implement pcibios_alloc_irq() and pcibios_free_irq() to dynamically
allocate and free PCI IRQs.

Remove mp_should_keep_irq(), which is no longer used.

[bhelgaas: changelog]
Signed-off-by: NJiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NThomas Gleixner <tglx@linutronix.de>