net: zynq_gem: Calculate clock dividers dynamically

Remove hard coded clock divider setting and use the Zynq clock framework
to dynamically calculate appropriate dividers at run time.

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