irqchip: Add Xilinx Interrupt Controller bindings

The Xilinx interrupt controller driver is being added such that
it can be used as a chained (cascaded) interrupt controller for
Zynq (and MicroBlaze later).

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>