segfault in xlinx_intc_fdt_auto_parent

[w1cmc]

If the interrupt controller is "realized" before the cpu, then DEVICE(first_cpu) returns NULL and we abort when we dereference it.

Not sure if some devicetree trickery can ensure that cpus are realized before interrupt controllers, but an example dts that exhibits the issue is at

https://forums.xilinx.com/xlnx/board/message?board.id=ELINUX&message.id=39127#M39127