MFC r256861:

MFP4: 223121 (FDT infrastructure portion)

Implement support for interrupt-parent nodes in simplebus.  The current
implementation requires that device declarations have an interrupt-parent
node and that it point to a device that has registered itself as a
interrupt controller in fdt_ic_list_head and implements the fdt_ic
interface.

Sponsored by:   DARPA/AFRL
Approved by:    re (gjb)