On the larger ppc64 machines we remap the interrupt numbers used by
the hardware/firmware to virtual IRQ numbers < NR_IRQS.  Up until now
we have used an array for the "real" (hardware) -> virtual IRQ number
mapping, but with new machines coming out that will have 24-bit
hardware IRQ numbers, this will break.  However, in fact it is only
the XICS interrupt controller which cares about this mapping.  This
patch moves that side of the mapping (real -> virtual) into the XICS
code and makes it use a radix tree.

On iSeries we have a similar issue, where the "real" IRQ numbers that
we need are in fact an encoding of the bus/device/function address of
the device.  This patch fixes iSeries to use the virt->real IRQ
mapping, allowing us to support larger iSeries machines.  This patch
also gets rid of the temporary hack that Stephen Rothwell submitted.

On machines with OpenPIC (including the G5) the mapping is explicitly
1-1, and that hasn't changed.  For other machines this patch cleans up
and simplifies the code that sets up the virtual->real mapping.

I have tested this code and verified that G5, pSeries and iSeries boot
and run correctly with this patch.