Add method for binding interrupt to CPU in PIC interface (currently unused)
and mechanism for unified caching and translating FDT cells describing
interrupt to actual IRQ number through PIC kobj interface.
Don't register as an interrupt controller until after resource allocation
and all the other up-front things that could fail.
Automatically allocate an ic_intrs array for nexus when it registers;
nexus isn't described in the fdt data which usually triggers allocation.