ARM: dts: zynq: Add arm_global_timer nodes

The driver has been merged quite a while ago. Add the missing DT nodes
to use the arm_global_timer.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>