irqchip/xilinx: Try to fall back if xlnx,kind-of-intr not provided

The powerpc dts file does not have the xlnx,kind-of-intr property.
Instead of erroring out, give a warning instead. And attempt to
continue to probe the interrupt controller while assuming
kind-of-intr is 0x0 as a fall back.

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>