http://bugme.osdl.org/show_bug.cgi?id=2574

mp_parse_prt() and iosapic_parse_prt() used to allocate all
IRQs, whether devices needed them or not.  Some devices
failed because the this method enabled unused PCI Interrupt
Link Devices, which disrupted active link devices.

Now the PRT knowledge is pulled out of the arch
code and the IRQ allocation and IO-APIC programming
is done by pci_enable_device().
This is also a step toward allowing the addition
of new root bridges and PRTs at run-time.

The architecture supplies

 unsigned int
 acpi_register_gsi(u32 gsi, int edge_level, int active_high_low)

which is called by acpi_pci_irq_enable().  ACPI supplies
all the information from the PRT, and the arch sets up
the routing and returns the IRQ it allocated.