arm: dt: zynq: Add 'cpus' node

Add a 'cpus' node to describe the CPU cores of Zynq.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>