arm: zynq: timer: Remove unused #defines

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: John Linn <john.linn@xilinx.com>
Tested-by: Josh Cartwright <josh.cartwright@ni.com>