arm: zynq: timer: Replace PSS through PS

The acronym PSS is deprecated by Xilinx. The correct term, which is
also used in Xilinx documentation is PS (processing system).
This is just a search and replace:
 - s/PSS/PS/g
 - s/pss/ps/g

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Josh Cartwright <josh.cartwright@ni.com>