ARM: dt: zynq: Add labels to cpu nodes to allow overriding OPPs.

By adding labels to the cpu nodes in the dtsi, a dts that
includes it can change the OPPs by referencing the cpu0
through the label.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>